74 THE FIRST DAY. [CHAP. 



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 notochord. The hinder diverticulum of the 

 neural canal along the hne of the primitive groove is, moreover, 

 very considerable in the chick, and is not so soon obhterated as 

 in the goose. The incomplete passage in the chick arises at a 

 period when about twelve somites are present. The third 

 passage is formed in the chick dru-ing the third day of incuba- 

 tion. 



The anterior part of the primitive streak becomes con- 

 verted into the tail-swelling ; the groove of the posterior part 

 gradually shallows and finally disappears. The hinder part itself 

 atrophies from behind forwards, and in the course of the folding 

 off of the embryo from the yolk the part of the blastoderm where 

 it was placed becomes folded in, so as to form part of the ventral 

 wall of the embryo. The apparent hinder part of the primitive 

 streak is therefore in reality ventral and anterior in relation to 

 the embryo. 



Since the commencement of incubation the area 

 opaca has been spreading outwards over the surface of the 

 yolk, and by the end of the first day has reached about 

 the diameter of a sixpence. It appears more or less 

 mottled over the greater part of its extent, but this is 

 more particularly the case with the portion lying next 

 to the pellucid area ; so much so, that around the pel- 

 lucid area an inner ring of the opaque area may be 

 distinguished from the rest by the difference of its 

 aspect. 



The mottled appearance of this inner ring is due to 

 changes taking place in the mesoblast above the germi- 

 nal wall — changes which eventually result in the forma- 



