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EMBRYOLOGY OF THE CHICK 

 BLASTULATION 



The chick embryo remains in the morula stage for a very short 

 period, when there is a rearrangement of cells preliminary to the blastula 

 formation. First, a cavity forms beneath the blastoderm due to the 

 smaller central cells separating from the underlying yolk. The outlying 

 cells remain attached. This space is called the segmentation cavity or 

 blastocoel, while the marginal area of the blastoderm which remains 

 attached to the yolk is called the zone of junction. As soon as the seg- 

 mentation cavity is thus formed the embryo is said to be in the blastula 

 stage. . 



From Figure 259, which shows only the blastoderm and a portion 

 of the yolk (the yolk being about three feet in diameter at this magnifi- 

 cation), a good understanding of the difference a larger amount of yolk 

 makes in the blastula-formation may be had. 



In eggs with little yolk a definite morula or solid sphere of cells can 

 easily form, which may then develop into a hollow sphere or blastula. 

 But in eggs with a large quantity of yolk, as in the pigeon and the chick 

 (Fig. 258), the blastomeres are forced to grow on the surface of the 



SECTION AT e-c 1ft C 



Fig. 259. 



Diagrams to show various stages in the gastrulation of a bird embryo. In 

 the surface-view plans, the blastoderm is supposed to be transparent so the under- 

 lying structures may be located. A, surface view of blastoderm, just before 

 invagination ; B, surface view of blastoderm, invagination well advanced ; C, 

 surface view of blastoderm at end of gastrulation ; D, vertical section through 

 blastoderm of stage represented in A; The plane of section is indicated by the line 

 a-a in A. E, vertical section through blastoderm of stage represented in B. The 

 plane of the section is indicated by the line b-b in B. F, vertical section through 

 blastoderm of stage represented in C. The plane of the section is indicated by the 

 lines c-c in C. (From Patten, after Patterson's figures for the pigeon.) 



