BY WILLIAM A. HASWELL, M.A., D.SC. 593 



front of this there is a very distinct neurenteric canal, which is 

 readily discernible wlien the embryo is looked at from the ventral 

 •aspect. In front of it, where the notochord ends posteriorly, there is 

 a complete continuity of epiblast, mesoblast, and hypoblast, and 

 the notochord is continuous with the hypoblast. 



A s will be seen from the series of sections figured (plate XIV. 

 figs. 21-23), the passage is a very direct and open one, leading 

 from the posterior end of the completely closed neural canal 

 behind the extremity of the notochord (?4. ch.) into the enteric 

 cavity. The wall of the passage has the same structure as that 

 of the neural canal, but the passage cannot be regarded as strictly 

 a bending downwards of the posterior end of the neural canal, the 

 latter being continued backwards behind it, though only for a 

 very short distance. At this stage the notochord has become 

 separated from the mesial thickening of tha primitive streak, with 

 which it was at first continuous, by the intervention of the neuren- 

 teric canal, and its posterior end appears as a thickening of the 

 hypoblast, It remains separate from the floor of the medullary 

 canal, in front of the neurenteric passage, though it may be said 

 to pass into it round the sides of the latter. 



An embryo of a hundred and twenty-one hours, though in- 

 cubated for three hours longer than that just described, had 

 apparently scarcely attained the same stage of development, since 

 the posterior end of the medullary axis presented exactly the same 

 appearance as in the case of the embryo of a hundred and fifteen 

 hours ; and there was an evident, though very narrow, neurenteric 

 canal. 



The neurenteric canal above described is the equivalent of that 

 first described by Gasser in the goose, and subsequently noticed by 

 Balfour and by Hoffmann in the chick, of the first (more anterior) 

 of those described by . Braun in the duck and the wagtail," and of 

 the one described by the same author in the pigeon and fowl and 

 in MelojysittacK^s uiidulatus. 



