REPTILIA. 205 



behind the passage. The mesoblast, epiblast, and to some 

 extent the hypoblast, are now fused together in the axial line, 

 and in the middle of the fused mass is seen a narrow diverticulum 

 (lie] which is probably equivalent to the posterior diverticulum 

 of the neural canal in Birds (vide p. 164). 



The general features of the stage will best be understood by 

 an examination of the diagrammatic longitudinal section repre- 

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

 over the head of the embryo. The notochord (ch) is seen as an 

 independent 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 continued back to the opening of the passage, which thus becomes 

 enclosed in the medullary folds, and forms a true neurenteric passage 1 . 



It will be convenient at this point to say a few words as to what 

 is known of the further fate of the neurenteric canal, and the early develop- 

 ment of the allantois. According to Strahl, who has worked on Lacerta 

 vivipara, the canal gradually closes from below upwards, and is obliterated 



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

 the early Lacertilian development, more especially in what concerns the so-called 

 neurenteric passage. They believe this structure to be closed below, and to form 

 therefore a blind sack open externally. The open end of this sack they regard as the 

 blastopore an interpretation which accords with my own, but they regard the sack as 

 the rudiment of the allantois, and hold that it is equivalent to the invaginated archen- 

 teron of Amphioxus. I need scarcely say that I believe Kupffer and Benecke to have 

 made a mistake in denying the existence of the ventral opening of this organ. Kupffer 

 in a subsequent paper (No. 155) states that my descriptions of the structure of this 

 organ do not correspond with the fact. I have perfect confidence in leaving the 

 decision of this point to future observers, and may say that my observations have 

 already been fully confirmed by Strahl (No. 160), who has also added some observa- 

 tions on the later stages to which I shall hereafter have occasion to allude. 



