470 



DEVELOPMENT OF THE CHICK. 



place, and the cleavage of the egg or zygote is a division into 

 cells. Unlike the cleavage of most animal eggs, that of the 

 hen is partial or ' meroblastic ' owing to the great mass of 

 egg-yolk, which appears to prevent the formation of cells 

 except over the germinal disc. This layer of cells into 

 which the surface of the disc is transformed is at first a single 

 one, but by horizontal divisions soon becomes converted into 

 a double one. The disc is now spoken of as the * blastoderm.' 

 At or about this stage the egg is laid (figs. 76 and 80). 



2. The First Day : Formation of the Germinal Layers. 



The blastoderm at the time of laying is a circular patch, 

 about 3' 5 mm. in diameter, on the surface of the yolk. 

 Owing to its lower specific gravity the blastoderm is always 

 uppermost, however the egg be rolled over. It consists of a 

 marginal white rim, the area opaca, and a central circular, 

 more translucent portion, the area pellucida. 



E B ZL 



Fig. 80. Vertical section of the blastoderm and adjacent part of the 

 yolk of a Hen's Egg towards the close of segmentation. The 

 anterior edge ia to the right, the posterior edge to the left hand. 

 (After Duval.) x 25. 



A, blastocoel or segmentation cavity. E, epiblast. N', nucleus of blasto- 

 mere, which as yet is only incompletely separated from the yolk. VL, vacuole. 

 Y, yolk. ZL, one of the lower-layer cells or blastomeres. 



Sections of the blastoderm show that the disc of cells 

 is not uniform in structure. It consists of several layers. 

 Among the lowermost of these a cavity is formed by the 

 accumulation of fluid. As this expands, the cells above it 

 are thinned out to form an outer germinal layer or epiblast, 

 and a lower germinal layer or hypoblast (fig. 81). The cavity 



