when ejaculation occurs during intercourse approximately 200 million sperm or spermatozoa are deposited into the vagina they swim through the cervix propelled by whip-like motions of their tails or flagella after which muscular contractions of the uterus direct them to the uterine tubes this process usually takes between 30 minutes and 2 hours only around 200 spermatozoa will reach the secondary oocyte in the uterine tube and of these only one will fertilize it fertilization cannot occur until two processes have taken place capacitation and the acrosomal reaction these can take several capacitation is not fully understood but secretions from the uterus wall and uterine tube destabilize the plasma membrane surrounding the head of the spermatozoa or acrosome resulting in the membrane becoming more fluid which helps to prepare the spermatozoa for the events of fertilization the spermatozoa become hyperactive their flagella beat more frequently and their heads move laterally the capacitated spermatozoa moved through the corona radiata a dense layer of granulosa cells surrounding the oocyte and come into contact with the zona pellucida the zona pellucida expresses specific receptor proteins called zp3 which bind to proteins expressed in the heads of the spermatozoa the binding of zp3 triggers the acrosome reaction during which the enzymatic contents of the acrosome are released these enzymes help to digest a path through the zona pellucida allowing the spermatozoa to enter the perivitelline space and reach the plasma membrane of the secondary oocyte with which it fuses to ensure that only one spermatozoan penetrates the zona pellucida and fuses with the oocyte membrane fusion of the spermatozoan and oocyte membranes activates a fast and a slow block to polyspermy during fast blocked polyspermy after fusion the oocyte membrane depolarizes preventing other spermatozoa from fusing with it slow block to polyspermy is also stimulated by this depolarization during slow block to polyspermy a wave of intracellular calcium is released causing small cortical granules beneath the oocyte membrane to release their contents rendering zp3 inactive and making the zona pellucida impermeable upon the spermatozoan entering the oocyte undergoes meiosis ii and further develops into the female pro nucleus during this time the sperm develops into the male pronucleus and the two pronuclei fuse to form a single diploid nucleus or zygote after fertilization the zygote undergoes rapid mitotic division known as cleavage the first cleavage completed about 30 hours after fertilization produces two identical cells called blastomeres this cell division continues and by day three there is a cluster of 16 identical blastomeres this is known as the morula the morula enters the cavity of the uterus around day four at this point around the 32 cell stage a fluid known as uterine milk starts to penetrate the zona pellucida to nourish the blastomeres the blastomeres continue to divide and as more uterine milk enters the morula they develop a central fluid-filled cavity known as the blastocyst cavity or blastocyle around day five after the development of the blastocyst cavity the developing embryo is known as a blastocyst around this time the zona pellucida degenerates and the blastocyst hatches into the uterine cavity ready to implant into the uterine wall implantation occurs approximately six to seven days after fertilization but can only occur if the endometrial wall is sufficiently prepared by the correct levels of hormones the blastocyst usually implants in the posterior portion of the fundus of the uterus initially the blastocyst attaches loosely to the endometrial wall but this attachment becomes stronger as the blastocyst burrows into the endometrium and the endometrium becomes increasingly vascularized eventually the blastocyst is completely embedded within the endometrium at which point the endometrium becomes known as the decidua the regions of the decidua have specific names relative to the site of blastocyst implantation the area between the embryo and the stratum basalis is known as the decidua basalis the area between the embryo and the uterine cavity is the decidua capsularis and the remaining area is known as the decitua parietalis implantation of the blastocyst usually occurs six to eight days after fertilization by the end of day eight the blastocyst has burrowed into the endometrium of the uterus at this time it is composed of two main components the outer cell mass the trophoblast and the inner cell mass the embryoblast as the trophoblast makes contact with the endometrium it differentiates into two layers an inner cytotrophoblast and an outer syncyotrophoblast the embryoblast differentiates into a bilaminar embryonic disc composed of two cell layers the hypoblast and the epiblast soon after the embryonic disc has formed a cavity begins to appear between the epiblast and the cytotrophoblast known as the amniotic cavity cells originating from the hypoblast begin to migrate forming a thin membrane which covers the inner surface of the cytotrophoblast this is called the exocelomic membrane the exocelomic membrane and cells of the hypoblast together form the walls of the primitive yolk sac by day nine the blastocyst is completely embedded in the uterus wall at this stage of development the growth of the syncitiotrophoblast and cytotrophoblast is much quicker than the bilaminer embryonic disc small holes called lacunae begin to form in the syncitiotrophoblast as it continues to expand by day 12 the lacunae stop growing and fuse to form large interconnecting spaces called lacunar networks capillaries in the endometrium surrounding the developing embryo dilate forming maternal sinusoids as the syncytiotrophoblast continues to expand enzymes begin to erode the lining of the sinusoids and uterine glands allowing maternal blood and uterine secretions to flow into the lacunar networks establishing a uteroplacental circulation the blood and uterine secretions only come into close proximity to the embryo allowing the exchange of gases and metabolites around the same time a new population of cells appear between the inner surface of the cytotrophiblast and the outer surface of the primitive yolk sac known as the extra embryonic mesoderm large cavities begin to appear in the extra embryonic mesoderm these gradually fuse to form one single cavity called the chorionic cavity around 13 days after fertilization a large portion of the exocelomic cavity is pinched off forming a smaller cavity the secondary yoke sac by the end of the second week of development the chorionic cavity enlarges and the bilaminar embryonic disc is joined to the trophoblast by a band of extra embryonic mesoderm called the connecting stalk the future umbilical cord by the end of the second week of development the bilaminar embryonic disk consisting of the hypoblast and epiblast has formed throughout the third week of development this by laminar disk differentiates to establish three primary germ layers in a process known as gastrulation around 15 days after fertilization a thickened structure forms along the midline in the epiblast near the caudal end of the bilaminar embryonic disc this is called the primitive streak at this stage the formation of the primitive streak defines the major body axes of the embryo including the cranial end towards the head and caudal ends towards the tail as well as the left and right sides of the embryo at the cranial end of the embryonic disc the primitive streak expands to create a primitive node which contains a circular depression known as a primitive pit this depression continues along the midline of the epiblast towards the caudal end of the streak forming a primitive groove once formed cells of the epiblast migrate inwards towards the streak detach from the epiblast and slip beneath it into the interior of the embryo this process is known as invagination the first cells to invaginate through the primitive groove invade the hypoblast and displace its cells the hypoblast cells are eventually completely replaced by a new proximal cell layer which is referred to as the definitive endoderm by day 16 the majority of the hypoblast has been replaced the remaining cells of the epiblast are now referred to as the ectoderm and forms the most exterior distal layer some of the invaginated epiblast cells remain in the space between the ectoderm and newly formed definitive endoderm these cells form a germ layer known as the mesoderm once the formation of the definitive endoderm and mesoderm are complete epiblast cells no longer migrate towards the primitive streak throughout gastrulation the ectoderm continues to form from the cranial to the caudal end of the embryo establishing three distinct primary germ layers throughout the whole embryonic disc the gastrulation process is finally complete the first event of neuralation is the formation of a thickened area of cells called the neural plate the neural plate forms at the cranial end of the embryo and grows in a cranial to caudal direction the cranial or head end of the neural plate indicates the region of the future brain and the narrower caudal or tail end represents the future region of the spinal cord by the end of the third week of development the lateral edges of the neural plate become elevated and move together to form the neural folds the resulting space created by the folding of the neural plate is called the neural groove the neural folds fuse together and the neural plate transforms into the neural tube the precursor to the central nervous system fusion of the neural tube usually begins in the middle of the embryo extending in both cranial and caudal directions during the closure of the neural tube cells on the crest of the neural folds detach forming a new cell population called the neural crest these cells contribute to the formation of the peripheral nervous system once the neural tube has completely fused the process of neurallation is complete during the fourth week of development a period of rapid growth the embryo begins to change shape from a flat trilaminer disc into a cylinder a process known as embryonic folding embryonic folding occurs in two planes the horizontal plane and the median plane and is the result of differing rates of growth of the embryonic structures folding of the embryo in the horizontal plane results in the development of two lateral body folds folding in the median plane results in the development of the cranial and caudal folds folding in both of these planes takes place simultaneously resulting in the rapid development of the embryo the cylinder consists of three layers derived from the trilaminer embryonic disc these are the endoderm the innermost layer the ectoderm the outermost layer and the mesoderm located in between the endoderm of the trilaminar disc is mainly responsible for the formation of the gastrointestinal tract as embryonic folding continues the endoderm moves towards the midline and fuses incorporating the dorsal part of the yolk sac to create the primitive gut tube the primitive gut tube differentiates into three main parts the foregut mid gut and hindgut the foregut can be seen at the cranial or head end of the embryo it is temporarily closed by the oropharyngeal membrane which at the end of the fourth week of development ruptures to form the mouth the mid gut lies between the fore and hind gut and remains connected to the yolk sac until the fifth week of development as embryonic folding continues the connection to the yolk sac narrows into a stalk known as the vitelline duct the hindgut lies at the caudal or tail end of the embryo it is temporarily closed by the cloacal membrane which during the seventh week of development ruptures to form the urogenital and anal openings as a result of embryonic folding the major body plan is established and the three germ layers continue to differentiate giving rise to their own specific tissues and organ systems you