3024. Finally, there is a respiratory process on the body of the embryo itself through the branchial apertures on its neck; these in the Frogs and Salamanders still continue visible after their exclusion from the egg. This oxygen must be derived from the fœtal water.
3025. Without doubt also the vitelline vessels respire, and thus each vegetative system has its own process of respiration. The intestine respires through the vitelline vessels, the vascular system through the vessels of the chorion, the sexual system through those of the allantois, the integument through the branchial apertures. The lungs belong to, and perform respiration for the whole body.
Decay of the Developmental Organs.
3026. When all the organs are developed, the intestines are drawn together with the vitellus into the abdomen. The chick is still nourished several days after exclusion from the egg by the vitelline mass, which passes through the vitelline canal into the intestine. The vitelline membrane subsequently becomes flaccid, and finally disappears through maceration. In the Mammalia the umbilical vesicle separates at an earlier period from the intestine, and continues to lie in the umbilical cord.
3027. The secernent point of the vitelline membrane or of the umbilical vesicle is the cœcum.
3028. There are therefore two intestinal systems, which branch off in a bifurcated manner from the cœcum, namely, the sexual intestine and the small or truncal intestine.
3029. All embryos have originally umbilical herniæ, which do not originate through protrusion of the intestines out of the abdominal cavity, but through their entrance into the latter being retarded.
3030. The umbilical herniæ therefore indicate an earlier condition of the animal, which has originated through arrest of the development.
3031. At birth all the enveloping membranes die, and their point of liberation is called the navel or umbilicus.
3032. The animal has originated through the umbilicus, and through this has it respired.