if we look at a cross-section of an embryo of the Frog Xenopus we can see that at this point it is a ball of cells with a fluid filled cavity the cavity is the Blastoise seal and the embryo is currently in the blastula stage of development a blastula contains large yolk filled cells at the vegetal pole and smaller cells at the animal pole the three colors represent the three tissue layers that become defined early in embryogenesis yellow indicates endoderm red indicates mesoderm and blue indicates ectoderm at the beginning of gastrulation a few surface cells called bottle cells move into the interior of the embryo followed by other surface cells we can track the movement of cells into the embryo if we add dye to a few surface cells the movement of cells into the embryo creates a lip called the dorsal lip over which sheets of cells continue to move inside at the same time the ectoderm extends around the embryo surface in a process called a pibbly as gastrulation proceeds a cavity called the arc enter on forms while the blasts Aseel progressively shrinks the arc enter on is the primitive gut and is completely surrounded by endo dermal tissue the endoderm at the roof of the cavity originated from the outside of the embryo the cavity is continuous with the outside via the blastopore which eventually becomes the anus of the animal as the ectoderm extends around the embryo another set of bottle cells forms these cells migrate into the embryo and other surface cells follow them creating the ventral lip of the blastopore by the end of gastrulation the ectoderm has surrounded the embryo endoderm lines the inside and mesoderm lies between the two additionally the fates of specific regions have become determined the endoderm gives rise to the digestive and respiratory tracts and associated structures the mesoderm gives rise to the skeleton circulatory system muscles excretory system and most of the reproductive system the ectoderm gives rise to the skin sense organs and nervous system