136 



AEUEENTEHIC CANAL. 



In the goose a passage has been described by Gasser, which appears 

 when about fourteen or fifteen somites are present, and lasts till twenty- 



Fig. 104. Four teansvehse sections thkough the neubenteuic passage and 

 adjoining parts in a duck embeyo 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 hypohlastic opening of the neurenteric canal, and tbe 

 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. 

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



streak. 



three are formed. Behind its opening the medullary canal is continued 

 back as a small diverticulum, which follows the course of the primitive 

 groove and is apparently formed by the conversion of this groove into a 

 canal. It is at first open to the exterior, but soon becomes closed, and then 

 atrophies. 



In the chick there is a perforation on the floor of the neural 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- 

 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 o[tenings 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 



