288 THE GASTRULA OF THE SAUROPSIDA. 



I consider that the embryos of the Sauropsida have come to 

 occupy a central position in the blastoderm owing to the abbre- 

 viation of a process similar to that by which, in Elasmobranchii, 

 the embryo is removed from the edge of the blastoderm ; and 

 that the primitive streak represents the linear streak connecting 

 the Elasmobranch embryo with the edge of the blastoderm after 

 it has become removed from its previous peripheral position, as 

 well as the true neurenteric part of the Elasmobranch blastopore. 



This view of the nature of the primitive streak, which is 

 diagrammatically illustrated in fig. 175, will be rendered more 

 clear by a brief review of the early developmental processes in 

 the Sauropsida. 



After segmentation the blastoderm becomes divided, as in 

 Elasmobranchii, into two layers. It is doubtful whether there is 

 any true representative of the segmentation cavity. The first 

 structure to appear in the blastoderm is a linear streak placed at 

 the hind end of the blastoderm, known as the primitive streak 

 (figs. 175 C, /5/and 176, pr). At the front end of the primitive 

 streak the epiblast and hypoblast become continuous, just as 

 they do at the dorsal lip of the blastopore in Elasmobranchii. 

 Continued back from this point is a streak of fused mesoblast and 

 epiblast to the under side of which a linear thin layer of hypoblast 

 is more or less definitely attached. 



A further structure, best developed in the Lacertilia, appears 

 in 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 continu- 

 ation 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 me- 

 dullary 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. 162 

 164) of the neurenteric canal are developed. 



On the hypothesis that the Sauropsidan embryos have come 



