REPRODUCTION 



79 



The early development of the placental mammal presents many 

 perplexing features. It could be expected that the minute egg, unem- 

 barrassed by yolk, would revert to the relatively simple and direct methods 

 of early development which, for the most part, characterize Amphioxus. 

 But it does not. Mammalian stages precisely comparable to the blastula 

 and gastrula of Amphioxus or amphibians cannot be recognized. When 

 it comes to the formation of mesoderm, the laying out of the germ layers, 

 and the early shaping up of the embryo, the behavior of the mammal is 

 closely similar to that of a reptile or bird. This similarity exists in spite 

 of the absence of a large yolk mass in the mammal. These facts, taken 

 together with the fact that certain mammals which are in most respects 



lEC 



@c 



i£&"!SSs«l4^^ 



Vmes 'en 



Fig. 54. — Transverse section of the embryonic shield of a Rabbit at the stage 

 represented in Fig. 53. The section is taken at the position indicated by the line 

 6^-5 in Fig. 53. EC, ectoderm; EN , endoderm; MES, mesoderm; PG, primitive groove 

 of primitive streak. X175. (After Assheton.) 



obviously of primitive type produce large yolk-laden eggs whose mode of 

 development is essentially reptilian, point to what may be regarded as a 

 fairly safe interpretation of the conditions in placental mammals. 



In connection with the attainment of viviparity the yolk content of 

 the mammalian egg became reduced to almost nothing. Apparently, 

 however, the developmental behavior of the reptilian embryo had become 

 so strongly established in the protoplasm of ancestral reptiles and primi- 

 tive mammals that it persisted even though the reduction of yolk had 

 removed the immediate necessity for many of its peculiarities. The 

 many millions of years of primitive mammalian and of reptilian lineage 

 constituted a barrier quite impassable by any tendency for reversion to the 

 indefinitely more remote developmental methods of primitive Amphioxus- 

 like chordates. Yet something from these primitive chordates came on 

 down to mammals. If, as seems fairly certain, the reptilian primitive 

 streak is a modified amphibian blastopore and, further, if the axial thick- 

 ening in the early embryonic shield of a mammal is, as seems likely, a 

 modified reptilian primitive streak — even to the inclusion of a blastopore 

 so belated and so atrophied as to have completely lost its original sig- 

 nificance as the aperture of a gastrular invagination — then a heritage 

 from primitive chordates is to be recognized in the embryo of a modern 

 mammal. 



Unquestionably the yolk content of the chordate egg is much more 

 readily subject to evolutionary change than is the developmental mecha- 



