242 



MAMMALIAN GASTRULA. 



grows out the greater part of the mesoblast. At the front end of the 

 primitive streak the hypoblast and epiblast become continuous, 

 though a perforated neurenteric blastopore has not yet been detected. 

 All these Sauropsidan features are so obvious that they need not 

 be insisted on further. The embryonic evidence of the common origin 

 of Mammalia and Sauropsida, both as concerns the formation of the 

 layers and of the embryonic membranes, is as clear as it can be. 

 The only difficulty about the early development of Mammalia is 

 presented by the epibolic gastrula and the formation of the blasto- 

 dermic vesicle (figs. 178 and 179). That the segmentation is a com- 

 plete one is no doubt a direct consequence of the reduction of the 

 food-yolk, but the growth of the epiblast cells round the hypoblast 



and the final enclosure of 

 the latter, which I have 

 spoken of as giving rise to 

 the epibolic gastrula, are 

 not so easily explained. 



It might have been 

 supposed that this process 

 was equivalent to the 

 growth of the blastoderm 

 round the yolk in the 

 Sauropsida, but then the 

 blastopore ought to be 

 situated at the pole of the 

 egg opposite to the em- 

 bryonic area, while, accord- 

 ing to Van Beneden, the 

 embryonic area corresponds 

 approximately to the blas- 

 topore. 



Van Beneden regards 

 the Mammalian blastopore 

 as equivalent to that in 

 the Amphibia, but if the 

 position previously adopted 

 about the primitive streak is to be maintained, Van Beneden's view 

 must be abandoned. No satisfactory phylogenetic explanation of the 

 Mammalian gastrula by epibole has in my opinion as yet been offered. 

 The formation of the blastodermic vesicle may perhaps be ex- 

 plained on the view that in the Proto-mammalia the yolk-sack was 

 large, and that its blood-vessels took the place of the placenta 

 of higher forms. On this view a reduction in the bulk of the ovarian 

 ovum might easily have taken place at the same time that the 

 presence of a large yolk-sack was still necessary for the purpose of 

 affording surface of contact with the uterus. 



Fig. 179. Babbit's ovum between 70- 



HOUES AFTEB IMPREGNATION. (After E. Van 



Beneden.) 



bv. cavity of blastodermic vesicle (yolk-sack) ; 

 ep. epiblast; hy. primitive hypoblast; Zp. mu- 

 cous envelope. 



