DEVELOPMENT OF THE PLACENTA. 159 



the whole outer surface of the egg is covered with a "dense 

 forest of tufts (Fig. 134). 



These hollow tufts are now penetrated from within by 

 the branching blood-vessels, which originate from the in- 

 testinal fibrous layer of the allantois, and which contain 

 the blood of the embryo, introduced through the navel vessels 

 (Fig. 198, ckz). On the other hand, dense networks of 

 blood-vessels develop in the mucous membrane, which 

 lines the inner surface of the uterus, particularly in the 

 neighbourhood of the depressions into which the chorion- 

 tufts penetrate {'plu). These vascular networks receive the 

 blood of the mother introduced through the uterus vessels. 

 The whole mass of these two sets of vessels, which are here 

 most intimately connected, together with the connecting 

 and enveloping tissues, is called the placenta, or " vascular 

 cake." Properly speaking, the placenta consists of two 

 quite difierent, though closely connected, parts ; internally, 

 of the embryonic placenta (placenta foetalis, Fig. 198, chs), 

 and externally of the maternal placenta (placenta uterina. 

 Fig. 198, plu). The latter is formed by the uterine mucous 

 membrane and its blood vessels : the former by the 

 secondary chorion and the navel vessels of the embryo. 



The mode in which these two "vascular cakes" com- 

 bine to form the placenta, as well as the structure, form, 

 and size of the latter, differs much in different Placental 

 Animals, and affords valuable data for natural classification, 

 and hence also for the tribal history of the whole sub-class. 

 The latter is primarily divisible into two main divisions, 

 based on these differences : the lower Placental Animals, 

 which are called Indecidua, and the higher Placental 

 Animals, or Deciduata. 



