REPTILIA. 169 



plate with a shallow groove i^ng), well-developed raesoblastic plates 

 {mep), already divided into somatic and splanchnic layers, and a com- 

 pletely formed notochord independent of the hypoblast {hy). In the 

 next section (B), taken justin front of the primitive streak, the notochord 

 is attached to the hypoblast, and the medullary groove is deeper ; while 

 in the section following (C), which passes through the front border 

 of the primitive streak, the notochord and hypoblast have become 

 fused with the epiblast. The section behind (D) shews the neurenteric 

 passage leading through the floor of the medullary groove and through 

 the hypoblast {ne). On the right side the mesoblastic plate has be- 

 come continuous with the walls of the passage. The last section (E) 

 passes through the front part of the primitive streak behind the 

 passage. The mesoblast, epiblast, and to some extent the hypoblast, 

 are now fused together in the axial line, and in th^ middle of the 

 fused mass is seen a narrow diverticulum {ne) which is probably 

 equivalent to the posterior diverticulum of the neural canal in Birds 

 {vide p. 136). 



Fig. 127. Diagrammatic longitudinal section of an embryo op Lackrta. 

 pp. body cavity, am. amnion; ne. neurenterix3 canal; ch. notochord; hy. hypoblast; 

 ej). epiblast of the medullary plate ; pr. primitive streak. In the primitive streak all 

 the layers are partially fused. 



The general features of the stage will best be understood by an 

 examination of the diagrammatic longitudinal section represented 

 in fig. 127. In front is shewn the amnion (am), growing over 

 the head of the embryo. The notochord {ch) is seen as an in- 

 dependent cord for the greater part of the length of the embryo, but 

 falls into the hypoblast shortly in front of the neurenteric passage. 

 The neurenteric passage is shewn at ne, and behind it is the front 

 part of the primitive streak. 



It is interesting to notice the remarkable relations of the notochord 

 to the walls of the neurenteric passage. More or less similar relations are 

 also well marked in the case of the goose and the fowl, and support the con- 

 clusion, deducible from the lower forms of Vertebrata, that the notochord is 

 essentially hypoblastic. 



The passage at the front end of the primitive streak forms the posterior 

 boundary of the medullary plate, though the medullary groove is not at 

 first continued back to it. The anterior wall of this passage connects 

 together the medullary plate and the notochordal ridge of the hypoblast. 

 In the stage represented in fig. 126 and 129 the medullary groove has 

 become continvied back to the opening of the passage, which thus becomes 

 enclosed in the medullary folds, and forms a true neurenteric passage'. 



1 Kupffer and Benecke (No. 154) give a very different account from the above of 



