THE BLASTOPOEE. Ill 



the yolk-plug being separated from each other by the narrow 

 circular groove which leads into the mesenteron. In the im- 

 medi ately su cceeding stages the blastopore becomes grea tly- 

 reduced in size, though still retaining its circular outline 

 (c/. Figs. 52, 54, 55). This reduction is effected, not by con- 

 traction of the whole circumference of the blastopore, but by 

 a folding toge ther, or concrescence, of it s li ps in the median 

 plane, beginn ing at the lower or ventral margin and proce eding 

 up wards tow ards the do rsal marg in, the line of fusion be ing 

 marked by a faint vertical groove on the surface of the embryo 

 (c/.¥ig. 58, A and B). 



At the lip of th e blastopo re, round its entire circumference , 

 the three germinal layers , e piblast, mesoblast, and hypoblast , 

 are in^istinguishably fused together (Figs. 54, 55) ; the sepa- 

 ration between the layers first appearing a little distance beyond 

 the margin of the blastopore. As the lips of the bl astopore 

 meet and uni te fr om below u pw ards, in the manner descr ibed 

 above, a vertical band is produced by their union, at the hinder 

 end of the embryo, in which the three germinal layers are fused. 

 Thi& band is spoken of as the primitive streak ; and the faint 

 median groove, already described (^iig. 08, a, C), which runs 

 along it, and marks the line of union of the right and left lips 

 of the blastopore, is named the primitive g rnnv e. 



The primitive streak and primitive groove are comparatively 

 inconspicuous features in the frog embryo, but are much more 

 prominent in the chick and the rabbit. They are probably 

 to be regarded as secondary rather than as essential characters, 

 and as associated with the great distension which the egg has 

 undergone in consequence of the number of yolk-granules 

 imbedded in its substance. 



The further development of the primitive streak, and the 

 ultimate fate of the blastopore, will be described in a later part 

 of this chapter. 



The redu ction in size of the blastopore, caused by tbe c.nri- 

 crescence of its lips, gives rise to a c orresponding diminu tion 

 of t he yolk^lug (of. Figs. 52, 54, y f) : and at the close of th e 

 peri od now being described t^^'' witTidrawa rmmplptply frnm the 

 anrfane nf thf^ PTnbryO (Fir '^'^) . 



