240 THE GASTRULA OF THE SAUROPSIDA. 



the form of a circular passage perforating the blastoderm at the 

 front end of the primitive streak (fig. 176 ne). This passage is 

 bounded anteriorly by the layer of cells forming the continuation of 

 the hypoblast into the epiblast. 



In the next stage the medullary plate becomes formed in front of 

 the primitive streak (fig. 175 C), and the medullary folds are continued 

 backwards so as to enclose the upper opening of the passage through 

 the blastoderm. On the closure of the medullary canal (fig. 177) this 

 passage leads from the medullary canal into the alimentary tract, and 

 is therefore the neurenteric canal ; and a post-anal gut also becomes 

 formed. The latter part of the above description applies especially to 

 the Lizard: but in Chelonia and most Birds distinct remnants (vide 

 pp. 135 and 136) of the neurenteric canal are developed. 



On the hypothesis that the Sauropsidan embryos have come 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 described receive a satis- 

 factory 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 

 coalesced edges of the blastopore ; and the growth outwards from it 

 of the mesoblast is probablj^ a renmant of a primitive dorsal in- 

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



^/^ n^j 



PP 



Fig. 176. Diagrammatic longitudinal section of an embryo of Lacerta. 

 pp. body-cavity ; am. amnion ; ne. neurenteric canal ; ch. nctocbord ; hy. hypoblast ; 

 ep. epiblast ; pr. primitive streak. In tlie 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, (1) the neurenteric canal, (2) the primi- 

 Itive streak behind this, (3) the blastopore at the pole of the yolk- 

 sack opposite the embryo. 



Mammalia. The features of the development of the placental 

 Mammalia receive their most satisfactory explanation on the hypo- 

 thesis that their ancestors were provided with a large-yolked ovum 



