chapter 5 lecture 5a this will talk about the different types of eggs based on their yolk abundance and distribution and cleavage patterns first let's start off reviewing the difference between an amniotic eggs and amniotic eggs so in the last chapter we talked about what it meant to be an amniote and the specific organisms that were going to be amniotes were going to be reptiles birds and mammals and to be an amniote that meant that there was going to be an amniotic membrane that surrounded an embryo during development it didn't say anything about a shell it just said there was going to be an amniotic membrane surrounding an embryo as it developed and that amniotic membrane was going to be a fluid-filled sac and that fluid-filled sac prevented the desiccation or drying out of that embryo which helped reduce the dependency on that moist or aquatic habitat so it basically freed up that amniote to successfully move from an aquatic habitat to a terrestrial habitat this is like the one major feature that made it possible for organisms to leave the aquatic habitat and successfully transition and live on or on land or in a terrestrial habitat so what it meant to be an an amniote remember that in the past we've talked about the word a or an if you put a in front of something that prefix means without okay so an an amniote means without an amnion because if it meant if an amniot had an amniotic membrane an an amniote will not have an amniotic membrane so anything that is an an amniote includes a lot of the invertebrates and then the fission amphibians okay so as far as vertebrates go it includes sufficient amphibians there is going to be no amniotic membrane that surrounds that embryo as it develops so that means you're going to have to have an aquatic or moist habitat for those eggs to develop otherwise the embryo inside that egg is going to dry out or desiccate so that amniotic egg is the all-important structure that was developed that allowed for organisms to successfully transition to a terrestrial habitat from an aquatic habitat and it will have four extra embryonic membranes which we'll talk about next so the past slide we talked about the amniotic egg is what allowed those tetrapods to move onto terrestrial habitats and allowed to free themselves up from water for reproduction or egg development so they no longer had to return to an aquatic habitat to reproduce and they no longer had to return to an aquatic habitat for their embryo to develop so that amniotic egg comes with four extra embryonic membranes and we're going to start talking about the amnion first because that's what this egg is named for the presence of that amnion so the amnion is going to be the um sac that will completely surround your embryo and it will be a fluid fill sac that completely surrounds that embryo and it prevents the desiccation or drying out of that embryo it also is going to protect and insulate that embryo as it develops so then next we're going to move on to the chorion the chorion is going to be the outermost membrane and it completely surrounds the embryo as well so this chorion is going to serve as a porous membrane that allows for gas exchange between that embryo and the outside environment the air around it so it allows for gas exchange between the embryo and the environment but when you look at a mammal specifically a placental mammal it will become the fetal side of the placenta it will develop into and grow fingers into the endometrial lining of the uterus and become the fetal side of the placenta so now if we move on to let's say the elantoise we'll take the elantois next the alantoys is going to be a membrane but notice this membrane does not completely surround the embryo it is just attached to the umbilicus or the belly of the embryo and it's going to be able to hold on to any nitrogenous waste material produced by that embryo so it's going to store and hold the nitrogen waste to prevent it from building up to toxic levels and killing the cells that are making that embryo now in a mammal it will become the umbilicus or the umbilical cord so this alandtoys which is going to be the sac that holds nitrogen waste material that's a waste that's produced by the embryo when you get to a mammal it will develop into the umbilical cord that leads to the fetal side of the placenta so between the chorion producing the fetal side of the placenta and the elantois producing the umbilical cord leading from the embryo to that placenta you now have a network set up of how to exchange materials from the mother through that placenta to the embryo and from the embryo through that umbilical cord and placenta to the mother so they can trade materials back and forth then finally we're going to look at the ox and the yolk sac is going to be another membrane but this membrane is not going to be attached um all right i shouldn't say it it's going to be it will not completely surround the offspring it will only be attached to the umbilicus or the belly of that embryo again so your yolk sac is going to hold the food source or the yolk for the developing embryo when you get to mammals it will be the first place that blood cells get produced because in a mammal they have a very tiny small yolk sac so it's not there for very long and it gets absorbed very quickly so what's left behind is that empty yolk sac so it will be the first place that blood cells get produced for this embryo because you'll start making blood cells in this embryo in that yolk sac long before bones get produced so ultimately later your blood cells will be produced in your bone marrow of your bones but first before the bones are produced those blood cells are going to get produced in that yolk sac for your embryo so this embryo is going to come with four different extra embryonic membranes and each of those extra embryonic membranes has a function the only two extra embryonic membranes that completely surround the embryo are the amnion and the chorion then your alantois and your yolk sac are only going to be attached to the ventral region of that offspring or the embryo that's inside the amniotic egg eggs can be classified on their abundance of yolk and their distribution of yolk inside that egg so if we first start out with the example that's going to be our isoleuce at the leg usually individuals with an isolescent leg are going to be your echinoderms like the starfish and the sea urchin and you're going to see those in lab so that isolecityl egg is going to have a tiny amount of yolk very little yolk but that yolk is going to be evenly distributed so our first example over here for our isolecityl egg is going to have a small amount of yolk and notice it's in kind of scattered out and it's evenly distributed throughout that whole entire egg so then if we move on to your next egg the miso less of the leg that mesolectual egg is going to be found in amphibians so it's going to be like the frog egg you're going to look at in lab that mesoless of the leg is going to have a medium size or a moderate amount of yolk and it's going to be concentrated or moved down to one end of that egg called a vegetal pole so for your mesolocital egg the photo that's over here is going to have that moderate amount of yolk that medium amount of yolk that's kind of located down here at the vegetal pole end of that egg the vegetal pole is where you're going to concentrate the yolk and then the animal pole the opposite pole is going to be where you begin the production of your embryo then if we move on to your tiloless at the leg the tila lecithill egg is going to be something like your chicken birds and reptiles and those birds and reptiles that tilalus of the legs will have very very large amounts of yolk and it will also be concentrated down at the vegetal pole so your example over here of a telolectal egg example like your bird is down here at the bottom where you have a very large quantity of yolk that's sunk down and located down at the vegetal pole of that egg the last one is going to be that we talk about is going to be a centrioles at the leg and you're not going to see that in vertebrate comparative anatomy because the things that have a centrioles of the leg are organisms like insects which are not vertebrates so the centro lethal luscital egg had a large amount of yolk but it was concentrated in the very center of the egg so the example over here that you have is going to be that centrioles ethyl egg that had a large amount of yolk in the very center of the egg where your isolecitol egg had a tiny amount of yolk that was evenly distributed and then your meso less of the leg had a medium amount of yolk that was down at the vegetal pole and your telolescent egg had a lot of yolk that was located down at the vegetal pole the type of yolk and the distribution of yolk inside of that egg will predetermine how that egg can cleave or divide a cleavage furrow is a dividing furrow it divides one cell into two cells so we're going to have cleavage that's referred to as holoblastic cleavage and if you look at the term look at the term holo in first so holo means whole so if you have holoblastic cleavage holoblastic cleavage is going to be able to divide that cell completely into so when you have a holoblastic cleavage furrow form it will completely divide that egg into two cells and then it will completely divide those two cells into four cells and completely divide those four cells into eight cells so it's going to happen when you have eggs that have a little bit or a moderate amount of yolk so remember that the little bit of yolk was going to be an isolecityl egg and a moderate amount of yolk was going to be a meso-lecithill egg so eggs that had a tiny a moderate amount of yolk or an isolecityl egg they could go through holoblastic cleavage and those individuals that had mesolectal yolks or moderate amount of yolks could also go through holoblastic cleavage so when we look in lab it's going to be the sea urchins that you look at the isolectothyl eggs and it's going to be the frog embryo which were the mesolectolex notice that both the isolecityl and the mesolectal holoblastic cleavage are going to use something called radial cleavage which we'll talk about in a minute all right so there are different types of holoblastic cleavage but the ones that we're going to be concentrating on are radial and spiral so we will the next slide will deal with both the radial and spot radial versus spiral cleavage but they are both encountered during holoblastic cleavage all right so then the second type of cleavage planes that can be produced are called neuroblastic cleavage in neuroblastic cleavage you're not going to be able to divide the cell completely in half so when that egg starts to divide it's going to be incompletely divided because of the huge large amounts of yolk so mirablastic cleavage is going to be incomplete division of that egg it will be a moderate amount of cleavage and that's going to happen when you have eggs that have large quantities of yolk like a telolectal egg so individuals with large quantities of yolk are going to have neuroblastic cleavage and that's going to be those tila less of the legs those telolephasal eggs with that large amount of yolk won't allow the cleavage furrow to cleave all the way through that egg it is only going to cleave through the animal pole it will not cleave through the vegetal pole so you end up forming your animal cleavage furrows producing the embryo on a disc that sits on top of the yolk because the cleavage planes will only extend through the animal pole and will not extend through the vegetal pole where the yolk is so mirablastic cleavage is going to be in complete division it won't completely cleave or divide that egg it will only cleave or divide the cells at the animal pole not the vegetal pole the holoblastic cleavage we just discussed that was going to divide the cell or the egg completely in half with every cell division could be divided into two categories of radio cleavage or spiral cleavage and then rotational cleavage is going to be a modification of the radial and spiral cleavage so deuterostomes in general when we looked at deuterostome characteristics we said all deuterostomes have radial cleavage it's just a embryological pattern so if it's going to be a deuterostome it will exhibit radio cleavage if it was a protostome it was going to exhibit spiral cleavage so the majority of the organisms that we're looking at especially in comparative vertebrate zoology since all vertebrates are going to be deuterostomes they will all exhibit radial cleavage spiral cleavage was only going to be in the organisms that were protostomes that we looked at back in general zone rotational cleavage is kind of a modification between radial cleavage and some spiral cleavage and we'll look at that on the next slide mammals have rotational cleavage and we are deuterostomes so we have a modification of that radial cleavage radial cleavage is when the cells are going to divide at right angles to each other so when you have your first cleavage that separates the cell into an animal in a vegetal pole and at that point the next cleavage furrow that forms will divide that embryo into two separate cells and it will make a cell on one side and a cell on the other side each will have a vegetal and an animal pole so you will continue dividing and each of these is going to set up axes an anterior posterior and a right and left axis so that you get bilateral symmetry and you're going to go from a colon a whole cell division holoblastic cell division where you're going to go from one cell into two cells two cells into four cells four cells into eight cells eight cells into sixteen cells so you're gonna make a solid ball of cells referred to as a marula so at this point you can tell the difference in the type of animal it's going to be based on what happens to those cells that are produced in that marula at that time so realize that radial cleavage is going to divide the cells and they're going to divide them up and stack them up into nice rows and nice columns and they're very loosely arranged and loosely packed so at any one time you can separate them away from each other and so instead of having let's say an 8 cell embryo you could separate those eight cells into two separate four cell embryos so identical twins can be produced from radial cleavage okay now spiral cleavage is what we looked at in your protostomes and that's where every single cell division would happen at a very precise angle and then once that cell division happened the cells would be rotated around to the right at a very precise angle and with every cell division that happens every one of those cells would then divide at that same angle and rotate off to the right so once that first cell cleavage furrow forms every cell in there the fate of every cell in spiral cleavage is predetermined as to whether it will ultimately lay in mesoderm ectoderm or endoderm so its fate is predetermined so anything that is going to be spiral cleavage is also called determinant cleavage because it's fate of the cells are predetermined as to what they can develop into but when you look at radio cleavage radial cleavage is referred to as indeterminate because those cells are going to divide divide again and divide again in rows and columns and they're loosely packed and they can separate easily and then continue on through cell division so their fate is not predetermined so that's the reason they can make the twins they have cells that are stacking up in nice neat rows loosely arranged and the cell fade is not predetermined so it's called radial cleavage that is also indeterminate cleavage during lab you're going to look at sea urchin embryos so you'll see radial cleavage that's holoblastic cleavage that is an isolescent leg you are also going to look at a frog which is going to be holoblastic cleavage that has a mesological moderate amount of yolk and you're also going to look at a chick which is going to be a mirror blastic example of cleavage because it's a tilalus of the leg with a huge amount of yolk so when we're looking at the frog first the frog's cleavage planes are going to be at right angles to each other but they're kind of up towards the animal pole so remember the first cleavage fur that forms divides the organism into an animal pole and a vegetal pole the vegetal pole is going to be the place where the yolk is concentrated and the animal pole is going to be the end where the majority of the cytoplasm is concentrated and that's where the animal is going to start to develop the embryo is going to start to develop that doesn't mean there won't be cleavage furrows that go all the way through that vegetal pole it's just that they cut the animal pole more readily and more easily and more thoroughly than they cut through the vegetal pole yolk so if you've got a frog egg it is a mesolecityl egg with a moderate amount of yolk that's located down at a vegetal pole so since it is going to be mesolectical and have a moderate amount of yolk the cleavage planes will be able to penetrate that yolk and they will ultimately produce as those cleavage furrows ultimately penetrate the animal pole they will eventually move down and divide up those vegetal pole cells as well and you're going to end up with an embryo that is called a blastula so the blastula is going to be the embryo stage that is a hollow ball of cells the hollow cavity in the very center will be the blastocell the actual embryo itself at that stage is called the blastula stage the chick is going to look a little bit different because when those cells start to divide the cleavage planes cannot penetrate the yolk all the way because this was a telo less at the leg there was too much yolk so this time the cleavage furrows will only cleave or only divide the cells that are on the animal pole and it's going to sit on a little tiny disc on top of that huge amount of yolk so in place of making a blastula what's going to happen is the embryo cells that are dividing on that little disc at that animal pole are producing something referred to as a blastoderm the blastoderm that little disk of embryonic cells that sit on top of the yolk and the chick embryo egg is going to be the equivalent of the blastula embryo stage in the frog egg okay so the blastula is a three-dimensional embryo in that egg where the blastoderm is still a three-dimensional embryo but it's going to form as a disc that sits on top a flat disc that sits on top of the yolk of the egg and it continues the embryo will get bigger and bigger and bigger as the yolk is consumed during the production of that egg cleavage in mammal is unique unto itself it's called rotational cleavage which is a modification on our radial cleavage remember we said that mammals are deuterostomes so they're going to have radio cleavage they just have a moderation of that radial cleavage called rotational so if we first look at what normal radial cleavage is like in your echinoderm or your amphibian that you looked at remember that you have a cleavage plane form and it divides one cell into two cells and then you're going to have a second cleavage plane form and it divides those two cells into four cells and they stack up in nice neat rows and columns so if i count the cells here and mark them you're going to have one cell you can have two cells three cells and four cells so you got all four cells there so when you look at cleavage furrows that form in rotational cleavage when you have your first cleavage furrow form it will divide the cells into cells that are on the right and cells that are on the left however when you do your second cleavage furrow okay instead of having that cleavage furrow form and divide these cells in that direction you're going to have the cleavage furrow form and it's going to divide the cells in this direction so you have two cells that are sitting beside each other and now you've got two cells where one is sitting on top of the other so it's a little bit different they're going to rotate so the next cell division that happens you're going to have some that are going to sit side by side and the next cell division that happens to those cells they'll be sitting one on top of each other so you rotate whether you're splitting them into columns and rows so it's a modification of that radial symmetry that's going to form so this is to help you in lab this is a wrap up of the different types of eggs based on their yolk abundance and distribution and their cleavage patterns each of these three organisms you're going to look at the eggs too and the development patterns too in lab when you come to lab so if we start over here with a sea urchin that sea urchin is going to have an isolecital egg with a small amount of yolk which means it can have holoblastic cleavage that completely divides that egg in half and it will use radial cleavage and divide it right angles when we look at the frog the frog egg will have a mesoless of the leg with a moderate amount of yolk that's located down at the vegetal pole it will also be able to use holoblastic cleavage and cleave the entire egg in half even eventually down at the yolk not just the animal pole but also at the yolk vegetal pulp and it's also going to be radial cleavage all right and then we're going to look at a chicken egg the chicken egg is the one that is a tila less of the leg it has a huge amount of yolk and there's so much yolk and it is concentrated down at the vegetal pole that there has to be neuroblastic cleavage used because there's so much yolk that the cleavage furrows can only penetrate and divide up the cells that are in the animal pole not the yolk cells that are down in the vegetal pole and there's also going to be the use of radio cleavage so notice that every one of these are going to use radial cleavage and that's because every one of these are going to be deuterostomes all of these vertebrates are deuterostomes so your frog and your chicken vertebrate organism or deuterostomes your sea urchin is a kind an echinoderm it's also going to be a deuterostome so they all use radial cleavage they do however use different types of cleavage furrows holoblastic if they have small immediate amounts of yolk but miroblastic if they have enormous large amounts of yolk and that's where iso miso versus telolephal comes in and describes the amount of yolk that's inside that egg alright