452 



EMBRYOLOGY OF THE CHICK 



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Fig. 261. 



A to >. Diagrams illustrating the idea of 

 confluence (concrescence) as applied to the chick. 

 The central area bounded by the broken line rep- 

 resents the area pellucida ; external to this is the 

 area opaca, showing the germ wall (G. W.), 

 zone of junction (Z.J.), and margin of over- 

 growth (M.O.). m.n., Marginal notch. 



E to G, Diagrammatic relations of the germ 

 layers at the time the primitive streak is formed 

 by concrescence of the blastoporal margins. E, 

 section of stage B ; F, section of stage D ; G, 

 section through blastoderm of a 16 hour chick 

 embryo. (A to D from Lillie's "The Development 

 of the Chick," by permission of Henry Holt & 

 Co.) 



forms are brought about by an 

 inpushing of the outer layer of 

 the blastula as one indents a 

 rubber ball. This forms a two 

 walled (ectodermal and ento- 

 dermal) cup with a cavity in 

 the center called a gastrocoele. 

 The opening itself is known as 

 the blastopore (Fig. 260). 



In birds with a large 

 amount of yolk, the blastula 

 cannot indent completely in- 

 to the blastocoele, due to the 

 disc-shaped blastoderm not be- 

 ing a true hollow sphere. The 

 very small blastocoele formed 

 between the blastoderm and 

 the yolk, allows but little in- 

 folding. The blastopore in the 

 case of an indented sphere is 

 relatively large. In the chick 

 there is but a tiny blastocoele, 

 while the blastopore is but a 

 small crescent-shaped slit at 

 the margin of the blastoderm 

 (Fig. 251, C). This slit is to 

 be thought of, however, as 

 similar to the regular round 

 opening in simpler forms, 

 which has been pushed to- 

 gether by the yolk not yielding. 

 The infolding entoderm is also 

 naturally compressed and flat- 

 tened by the tiny blastocoele 

 into which it can grow. In 

 fact, the lower layer of the in- 

 folding entoderm seems to be 

 prevented from growing nor- 

 mally by the unyielding yolk, 

 and so is broken and lies on the 

 yolk as scattered cells. These 

 scattered cells then shortly dis- 

 appear so that the yolk itself 

 forms the floor of the gastro- 

 coele. 



