PLACENTA. 557 



respectively the epiblast, mesoblast, and hypoblast. From the 

 epiblast become developed the epidermis and its appendages, and 

 the cerebro-spinal axis, from the hypoblast the mucous membrane 

 of the alimentary canal, while the mesoblast gives rise to the 

 intervening organs and tissues. 



A linear indentation, the primitive groove, appears on the 

 epiblast, and below and coincident with this the notochord, which 

 indicates the position of the vertebral column, becomes formed. 



The chief appendages and coverings associated with foetal life 

 are the umbilical sac, the amnion, chorion, and allantois, the 

 placenta, and the umbilical cord. 



The outer covering of the foetus is the chorion, a membrane 

 derived from the blastoderm and reflected over the fcetus ; the 

 placenta is formed by the connection of this covering with the 

 mucous membrane of the uterus. The inner covering is the 

 amnion, which contains a fluid, the liquor amnii. The allantois 

 or urinary vesicle is regarded as taking its origin from the 

 primitive intestine. Becoming vesicular in form to it proceed 

 the ducts of the Woljffia^i bodies, or primordial kidneys. 



The umbilical sac proceeds from the blastoderm ; it covers the 

 yelk, and like the allantois is not included in the amnion. 



PLACENTA. 



The blood of the foetus becomes purified, and charged with the 

 materials of nutrition, by being brought into close connection with 

 the blood of the mother, and this process is achieved in a struc- 

 ture called the placenta, an organ which contains capillary plexuses 

 derived from the umbilical arteries of the foetus, and others from 

 the uterine arteries of the mother ; it is situated between the 

 mucous membrane of the uterus and the external membranous 

 sac, the chorion, which completely envelops the foetus, to both 

 of which structures it is attached. 



The placenta assumes a variety of forms in the various classes 

 of animals. In the mare and sow it is diffused — i.e., it consists 

 of villi scattered over nearly the whole outer surface of the chorion, 

 and received into crypts or corresponding recesses in the uterine 

 mucous membrane. In the ruminant the placental villi are 

 collected into groups, forming a number of rounded or ovoid objects 

 termed cotyledons, hence this placenta is said to be polycoty- 

 ledonary. However in the Camelidce — "The foetal villi are 



