COMPARISON OF THE GERMINAL LAYERS. 289 



to occupy their central position, owing to an abbreviation of a 

 process analogous to the linear closing of the blastopore behind 

 the embryos of Elasmobranchii, all the appearances above describ- 

 ed receive a satisfactory explanation. The passage at the front 

 end of the primitive streak is the dorsal part of the blastopore, 

 which in Elasmobranchii becomes converted into the neurenteric 

 canal. The remainder of the primitive streak represents, in a 

 rudimentary form, the linear streak in Elasmobranchii, formed by 

 the coalesced edges of the blastoderm, which connects the hinder 

 end of the embryo with the still open yolk blastopore. That it 

 is in later stages not continued to the edge of the blastoderm, as 

 in Elasmobranchii, is due to its being a rudimentary organ. The 

 more or less complete fusion of the layers in the primitive streak 

 is simply to be explained by this structure representing the co- 

 alesced edges of the blastopore ; and the growth outwards from 

 it of the mesoblast is probably a remnant of a primitive dorsal in- 

 vagination of the mesoblast and hypoblast like that in the Frog. 



FIG. 176. DIAGRAMMATIC LONGITUDINAL SECTION OF AN EMBRYO OF LACERTA. 

 //. body cavity; am. amnion; ne. neurenteric canal; ch. notochord; hy. hypo- 

 blast; ep. epiblast; pr. primitive streak. In the primitive streak all the layers are 

 partially fused. 



The final enclosure of the yolk in the Sauropsida takes place 

 at the pole of the yolk-sack opposite the embryo, so that the 

 blastopore is formed of three parts, (i) the neurenteric canal, (2) 

 the primitive streak behind this, (3) the blastopore at the pole of 

 the yolk-sack opposite the embryo. 



Mammalia. The features of the development of the placen- 

 tal Mammalia receive their most satisfactory explanation on the 

 hypothesis that their ancestors were provided with a large-yolked 

 ovum like that of the Sauropsida. The food-yolk must be sup- 

 posed to have ceased to be developed on the establishment of a 

 maternal nutrition through the uterus. 



On this hypothesis all the developmental phenomena subse- 

 B. in 19 



