REPTILIA. 207 



and opened again. It has, in any case, the same relations as in the previous 

 stage. 



It leads from the end of the medullary canal (at the point where its 

 walls are continuous with the cells of the primitive streak) round the end 

 of the notochord, which here becomes continuous with the medullary cord, 

 and so through the hypoblast. The latter layer is still a flat sheet without 

 any lateral infolding ; but it gives rise, behind the neurenteric passage, to 

 a blind posteriorly directed diverticulum, placed in the body cavity behind 

 the embryo, and opening at the ventral face of the apparent hind end 

 of the primitive streak. There is very little doubt that this diverticulum 

 is the commencing allantois. 



At a somewhat later stage the arrangement of these parts has undergone 

 some changes. Their relations are shewn in the sections represented in 

 fig. 128. 



The foremost section (A) passes through the alimentary opening of the 

 neurenteric passage (ne). Above this opening the section passes through 

 the primitive streak (pr) close to its junction with the walls of the medullary 

 canal. The hypoblast is folded in laterally, but the gut is still open below. 

 The amnion is completely established. In the next section figured (B), the 

 fourth of my series, the gut is completely closed in ; and the mesoblast has 

 united laterally with the axial tissue of the primitive streak. Vessels to 

 supply the allantois are shewn at v. 



The three following sections are not figured, but they present the same 

 features as B, except that the primitive streak gets rapidly smaller, and the 

 lumen of the gut narrower. The section following (C) represents, I believe, 

 only the stalk of the allantoic diverticulum. This diverticulum appears 

 to be formed as usual of hypoblast (hy) enveloped by splanchnic mesoblast 

 (me), and projects into the section of the body cavity present behind the 

 embryo. Its position in the body cavity is the cause of its somewhat 

 peculiar appearance in the figure. Had the whole section been represented 

 the allantois would have been enclosed in a space between the serous mem- 

 brane (se) and a layer of splanchnic mesoblast below which has also been 

 omitted in fig. B 1 . It still points directly backwards, as it primitively does 

 in the chick, -vide fig. 123 A, and Gasser, No. 127, PL v. figs, i and 2. I do 

 not understand the apparently double character of the lumen of the allantois. 

 In the next section (not figured) the lumen of the allantoic stalk is larger, 

 but still apparently double, while in the last section (D) the lumen is 

 considerably enlarged and single. The neurenteric canal appears to close 

 shortly after the stage last described, though its further history has not 

 been followed in detail. 



1 Owing to the difficulty of procuring material I have only been able to prepare the 

 two sets of sections just described, and in the absence of a fuller series there are some 

 points in the interpretation of the sections which must remain doubtful. 



