PHYSIOLOGY. 485 



close upon it. Through this originates a saccular inver- 

 sion, as in the mesentery of the peritoneum, and the 

 vesicle separates into three divisions. In itself it is 

 ai union, the inverted part is the integument of the 

 embryo, the tube which united these the umbilical cord. 



2983. The amnion is thus the root or primary bladder 

 of the integument. 



2984. Through continuous oxydation blood-vessels are 

 developed upon the surface of the amnion, which finally 

 withdraw to constitute a special integument, which is 

 called chorion. Its vessels are in like manner repelled 

 from the fundus uteri, and prolonged into the inversion" 

 of the umbilical cord and the embryo. The chorion is J 

 thus the root or primary bladder of the vascular system. 



2 98 5. These two bladders, sacs, or cysts are the only 

 general ones which circumscribe or invest the entire 

 embryo, because there are only two general vegetative 

 systems, namely, the tegumentary and vascular. 



2986. The embryo has not originated freely in these 

 shut sacs, but only through their introversion ; it is itself 

 a portion of these sacs. 



2987. The embryo properly lies external to its en- 

 velopes, as the intestine does in respect to the mesentery. 



2988. Just as the two general vegetative systems have 

 been developed from primary cysts, so also are there sacs 

 for the two special vegetative systems, or the intestinal 

 and sexual;, but, on that very account, these cannot be 

 general sacs, nor any longer envelop the embryo. 



2989. At the entrance of the inversion of the umbi- 

 lical cord is situated a small vesicle, which divides and is 

 prolonged into the two intestines. It is therefore the root 

 or primary sac of the intestinal system, and is called in Man 

 the ' vesicula umbilical is,' in the Mammalia the ' tunica 

 erythroides/ and in oviparous animals the 'vitellus ' 



2990. In the same situation is placed another bladder, 

 which is prolonged into what has been called the ' urachus' 

 and the urinary bladder, and from which the primordial 

 kidneys, the true kidneys, and the sexual parts are de- 

 veloped by sacciform eversion. This sac is called the 



