260 



AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The blastoderm thus comes to be three-layered, both in front 

 and behind. 



The closure of the blastopore in a frog-embryo produces a small primitive 

 streak, in which the germinal layers are continuous. There can be no 

 doubt that the primitive streak in the chick-embryo is equivalent to a 

 closed blastopore of slit-like form. If the slit were to re-open it would 

 lead into the space figured at / in Fig. 75, and which = an archenteron. 

 The gastrula of the fowl, therefore, is very much modified, owing to (1) 

 closure of blastopore, (2) enormous accumulation of yolk on ventral side. 



Ao 



^Z-SGiZSS&r 



Fig. 77. SURFACE VIEW OF FIRST DAY (20 HRS.) CHICK (from Kolliker). 

 A.o, Area opaca, bounding A.p, area pellucida ; Pr, primitive streak ; 

 Af, head-fold. 



2. General Growth. The blastoderm is at first a watch-glass 

 shaped plate resting upon the yolk. It generally grows in all 

 directions, and ultimately completely encloses the yolk. This 

 enclosure is mainly effected by the area opaca. The body of the 

 embryo is formed in the area pellucida. This elongates (Fig. 77) r 



