KEPKUDUCTION 



57 



of a sauropsidan embryo. At the anterior end of this mammalian streak 

 is usually found a small pit or even a perforation extending through the 

 shield into the cavity of the blastodermic vesicle — very suggestive of an 

 abortive blastopore. It is along this mammalian primitive streak, as 

 in the similar sauropsidan structure, that rapid proliferation of cells 

 produces a mesoderm (Fig. 50) which progressively interpolates itself 

 between the already separated ectoderm and endoderm and spreads 

 eventually into all regions of the embryo. The mesoderm is at first a 

 continuous layer — unsegmented — and devoid of cavity. 



VMES 'en 



Fig. 50. — Transverse section of the embryonic shield of a rabbit at the stage repre- 

 sented in Fig. 49. The section is taken at the position indicated by the line 5-5 in 

 Fig. 49. EC, ectoderm; EN, endoderm; MES, mesoderm; PG, primitive groove of 

 primitive streak. X175. (After Assheton.) 



In a rabbit embryo the embryonic shield is established ordinarily 

 by the fifth day of development, the entire blastodermic vesicle then 

 having a diameter of about 1.5 mm. 



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 point 

 to the conclusion that the developmental behavior of the reptilian embryo 

 had become so strongly established in the protoplasm of ancestral reptiles 

 and primitive 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 rever- 

 sion to the indefinitely more remote developmental methods of primitive 

 Amphioxus-like chordates. 



Unquestionably the yolk content of the chordate egg is much more 

 readily subject to evolutionary change than is the developmental mecha- 

 nism of the germinal protoplasm. That mechanism can be changed. 



