COMPARISON OF THE GERMINAL LAYERS. 



291 



In the formation of the embryo out of the embryonic area the 

 phenomena which distinguish the Sauropsida from the Ichthyo- 

 psida are repeated. The embryo lies in the centre of the area ; 

 and before it is formed there appears a primitive streak, from 

 which there grows out the greater part of the mesoblast. At the 

 front end of the primitive streak the hypoblast and epiblast be- 

 come 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 com- 

 mon 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 develop- 

 ment of Mammalia is presented by the epibolic gastrula and the 



FIG. 179. RABBIT'S OVUM BETWEEN 70 90 HOURS AFTER IMPREGNATION. 



(After E. van Beneden.) 



bv. cavity of blastodermic vesicle (yolk-sack) ; ep. epiblast ; hy. primitive hypo- 

 blast ; Zp. mucous envelope. 



formation of the blastodermic vesicle (figs. 178 and 179). That 

 the segmentation is a complete one is no doubt a direct conse- 

 quence 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. 



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