XIV ELEMENTS OF VERTEBRATE EMBRYOLOGY 473 



placenta forms a broad band round the equator of the blastocyst — ^the 

 polar portions being clear of placenta. 



Apart from these differences in general arrangement there are in- 

 teresting differences in the degree of elaboration of the placenta in detail, 

 more especially as regards the degree of intimacy of union between the 

 embryonic and the maternal parts of the placenta. In the less highly 

 evolved condition such as occurs in many diffuse placentas the embryonic 

 villi simply fit into recesses or crypts of the uterine wall, lined by ordinary 

 uterine epithelium, from which at birth they are withdrawn. Such an 

 arrangement is not fitted for anything like so perfect an interchange 

 between the maternal and the embryonic blood-streams as the arrange- 

 ment seen in the Rabbit but the lesser degree of efficiency of the placenta 

 in this respect is compensated for by the presence in the uterine cavity .of 

 a nutritive fluid — uterine milk as it has been called, containing protein, 

 fat, and carbohydrate food-material secreted by the uterine epithelium, 

 as well as blood extravasated from the uterine wall — which is absorbed 

 by the trophoblastic covering of the blastocyst. 



The more highly evolved types of placenta involve extensive inter- 

 ference with the normal uterine lining and in such cases the lining is 

 shed and replaced by a new lining after each birth and it was formerly 

 customary to attach great importance to this fact in classifying the 

 various types of placenta, a sharp distinction being drawn between 

 (i) " deciduate " placentas in which such shedding of the uterine lining 

 with the placenta attached to it took place after birth and (2) " in- 

 deciduate " placentas where at birth the embryonic part of the placenta 

 was simply withdrawn, leaving the uterine lining comparatively uninjured. 



Apart from the readily understood absence of yolk, and its natural 

 consequences the diminution in size of the mammalian egg and its 

 reversion to the holoblastic type of segmentation, the most striking 

 feature in the embryology of the typical mammal is the development 

 of the body of the new individual from the " inner mass '' of cells. What 

 is the meaning of this inner mass of cells which we find in no vertebrate 

 outside the group of mammals ? How has it originated in evolution ? 



The answers to these questions appear to depend upon two facts, 

 (i) The bulk of the young developing vertebrate is controlled by 

 limiting factors, such as the rigid egg-shell in the case of a bird, or the 

 necessity of not interfering with the health of the containing maternal 

 body in the case of the mammal. (2) A certain amount of free space 

 has been made available in the interior of the mammalian blastocyst by 

 the disappearance of the yolk. 



As a result of these two conditioning factors the portion of blastocyst 



