UROGENITAL SYSTEM 229 



The human embryo is similar to all the other placentals in 

 deriving its nourishment from the placenta, which, strictly 

 speaking, is composed of two parts, the foetal placenta and the 

 maternal placenta, formed from the chorionic villi and uterine 

 mucous membrane respectively. 



Man, like the other choriata (Ungulates, etc.), shares with 

 the indeciduata the peculiarity of originally having the villi 

 distributed over the whole surface of the chorion, these villi, 

 without the interposition of a decidua, penetrating into the 

 mucosa of the uterus. 



But at a later stage this becomes altered, the villi over one 

 portion of the chorion disappear, and in the other part grow 

 proportionately thicker and coalesce so intimately with the 

 uterine wall that at birth a portion of this latter is torn away 



FIG. 118. Central nervous system of the human embryo at the beginning of the 

 seventh week, v, fore-brain (prosencephalon) ; z, between-brain (thalamen- 

 cephalon) ; m, mid-brain (mesencephalon) ; h, hind- brain (epencephalon) ; , 

 after -brain (medulla oblongata). 



and forms the so-called decidua. This occurs in beasts of 

 prey, rodents, apes and man. 



The placenta is disc-shaped in man, apes, insectivora and 

 rodents. The naked eye and microscopic appearances of the 

 human placenta and that of apes are very similar, for in both 

 cases the maternal blood vessels form great sinuses into which 

 the f octal villi with their innumerable branches enter. 



The blood relationship between man and the apes is further 

 borne out by the survival of certain foetal rudiments which can 

 be found at birth, namely, the atrophied yolk-sac persisting as 

 the umbilical vesicle, and the remains of the pedicle of the 

 allantois ; these rudiments have been shown by Selenka to 

 be present in the anthropoids, particularly the orang. 



Human embryos at the end of the first half of intra-uterine 



