164 



NEURENTERIC CANAL. 



which is not so marked as those in the goose or duck, and never results 

 in a complete continuity between the neural and alimentary tracts ; but 

 simply leads from the floor of the neural canal into the tissues of the 

 tail-swelling, and thence into a cavity in the posterior part of the noto- 



FlG. 104. FOUR TRANSVERSE SECTIONS THROUGH THE NEURENTERIC PASSAGE 

 AND ADJOINING PARTS IN A DUCK EMBRYO WITH TWENTY-SIX MESOBLASTIC 

 SOMITES. 



A. Section in front of the neurenteric canal shewing a lumen in the notochord. 



B. Section through the passage from the medullary canal into the notochord. 



C. Section shewing the hypoblastic opening of the neurenteric canal, and the 

 groove on the surface of the primitive streak, which opens in front into the medullary 

 canal. 



D. Primitive streak immediately behind the opening of the neurenteric passage. 

 me. medullary canal; ep. epiblast; hy. hypoblast; ch. notochord; pr. primitive 



streak. 



chord. The hinder diverticulum of the neural canal along the line of the 

 primitive groove is, moreover, very considerable in the chick, and is not so 

 soon obliterated as in the goose. The incomplete passage in the chick 

 arises when about twelve somites are present. It is regarded by Braun as 

 equivalent to the first formed passage in the duck, but I very much doubt 

 whether there is a very exact equivalence between the openings in different 

 types, and think it. more probable that they are variable remnants of a 

 primitive neurenteric canal, which in the ancestors of those forms persisted 

 through the whole period of the early development. The third passage is 

 formed in the chick (Kupffer) during the third day of incubation. In 



