292 



LEISURE-TIME STUDIES. 



consists of a longitudinal streak or depression formed in the 

 epiblast or outer of the three layers already mentioned. 

 When the development of the chick is studied stage by 

 stage, all the changes just described occur during the first 

 twelve hours of incubation. During the first day of the life 

 of the chick, certain other and highly important changes will 

 occur. A second groove will soon grow backwards (Fig. 55), 

 widening as it proceeds, and will well-nigh obliterate the 

 first or " primitive groove ; " and in a few hours more, that 

 is, towards the end of the first day, the edges (Figs. 54, 

 d d, and 55, d d), of this second groove will become more 

 prominent, will finally unite in the middle line, and will 



FlG. 55. Development of chick in various stages, A, u, c, D : rt/head ; b, tail-ex- 

 tremity ; c, primitive groove ; d, sides of primitive groove ; e, first traces of the 

 vertebrae ; f t primitive veins. 



thus convert the groove into a canal. This canal (Fig. 54, 

 c) represents the minute tube found in the centre of the 

 future spinal cord, which, as every one knows, is contained 

 within the spine itself. A further development of the front 

 portion of the young animal will produce the head-folds and 

 skull, with its contained brain (Fig. 56, f& 9 etc.), and the 

 growth downwards of other parts of the embryo will similarly 

 produce the great bulk of the body (Figs. 55, 56) with its 

 contained organs. In Fig. 55 the gradual formation of 

 the primitive groove (c) may be studied as represented in 



