AVE8. 135 



off of the hind end of the embryo from the yolk commences. Co- 

 incidently with the last-named changes the sides of the front part of 

 the primitive streak become thickened, and give rise to conspicuous 

 caudal swellings ; in which the layers of the embryo are indis- 

 tinguishably fused. The apparently hinder part of the primitive 

 streak becomes, as more particularly explained in the sequel, folded 

 downwards and forwards on the ventral side. 



This is a convenient place to notice remarkable appearances which 

 present themselves close to the junction of the neural plate and the primi- 

 tive streak. These are temporary passages leading from the hinder end of 

 the neural tube into the alimentary canal. They vary somewhat in different 

 species of birds, and it appears that in the same species there may be 

 several openings of the kind, which appear one after the other and then 

 close again. They were first discovered by Gasser (No. 127). In all cases 1 

 they lead round the posterior end of the notochord, or through the point 

 where the notochord falls into the primitive streak. 



If the primitive streak is, as I believe, formed of the lips of the blasto- 

 pore, there can be but little doubt that these structures are disappearing, 

 and functionless rudiments of the opening of the blastopore, and they thus 

 lend support to my view as to the nature of the primitive streak. That, in 

 part, they correspond with the neurenteric canal of the Ichthyopsida is clear 

 from the detailed statements below. Till their relations have been more 

 fully worked o\\t it is not possible to give a more definite explanation of 

 them. 



According to Braim (No. 120) three independent communications are to 

 be distinguished in Birds. These are best developed in the Duck. The first of 

 these is a small funnel-shaped diverticulum leading from the neural groove 

 through the hypoblast. It is visible when eight mesoblastic somites are 

 present, and soon disappears. The second, which is the only one I have 

 myself investigated, is present in the embryo duck with twenty-six meso- 

 blastic somites, and is represented in the series of sections (fig. 104). The 

 passage leads obliquely backwards and ventralwards from the hind end 

 of the neural tube into the notochord, where the latter joins the primitive 

 streak (B). A narrow diverticulum from this passage is continued forwards 

 for a short distance along the axis of the notochord (A, clt). After tra- 

 versing the notochord, the passage is continued into a hypoblastic diver- 

 ticulum, which opens ventrally into the future lumen of the alimentary 

 tract (C). Shortly behind the point where the neurenteric passage com- 

 municates with the neural tube the latter structure opens dorsally, and 

 a groove on the surface of the primitive streak is continued backwards 

 from it for a short distance (C). The first part of this passage to appear 

 is the hypoblastic diverticulum above mentioned. 



This passage does not long remain open, but after its closure, when the 

 tail-end of the embryo has become folded off from the yolk, a third passage 

 is established, and leads round the end of the notochord from the closed 

 medullary canal into the post-anal gut. It is shewn diagram matically in 

 fig. 10G, ne, and, as may be gathered from that figure, has the same relations 

 as the neurenteric canal of the Ichthyopsida. 



1 This does not appear to be the case with the anterior opening in Melopdttacus 

 nndulatus, though its relations are not clear from Braun's description (No. 120). 



