THE BLASTODERM. 235 



and, by rapid division, the cells become of nearly uniform size in 

 all parts of the blastoderm. 



The lower-layer cells become more sharply separated from 

 the yolk, a space, filled with fluid, appearing beneath them, be- 

 tween the blastoderm and the yolk. This space, the subger- 

 minal cavity (Fig. 106, BV), is sometimes spoken of as the seg- 

 mentation cavity ; a name, however, which ought to be restricted 

 to the narrow chink between the epiblast and the lower-layer 

 cells (Fig. 105, B), which is clearly visible in the early stages, 

 but becomes practically obliterated before the egg is laid. 



Round its margin, new cells are still being cut out of the 

 yolk, and added on to the blastoderm. Some of the cells which 

 arise in this way, and lie between the edge of the blastoderm 

 and the yolk, are markedly larger than any of the others, and 

 are spoken of as formative cells (Fig. 106, ZF). 



FIG. 100. Vertical section of the blastoderm and adjacent parts of the yolk 

 of a Hen's Egg at the time of laying, but before the commencement of 

 incubation. The anterior edge of the blastoderm is to the right, the 

 posterior edge to the left side of the figure. (After Duval.) x 25. 



BV, subgermiual cavity. E, epiblast. TT. hypoblast. N', nucleus iu yolk, round 

 which a cell will be formed later. Y. yolk. ZF, formative cell. ZL, lower-layer cells. 



2. The Blastoderm. 



a. The condition of the blastoderm at the time of laying- of 

 the egg. The actual stage of development reached when the 

 egg is laid depends on the length of time the egg remains in 

 the uterus ; and this we have seen is subject to considerable 

 variation. The following description will apply to an average 

 case. 



Naked-eye examination shows the blastoderm (Fig. 97, BA) 

 to be a small circular patch, about 3'5 mm. in diameter, on the 

 surface of the yolk : owing to its less specific gravity, the blasto- 

 derm is always uppermost, however much the egg be rolled over. 

 The blastoderm consists of a marginal white rim, the area opaca. 

 thickest at the posterior edge of the blastoderm (Fig. 106, ZF) ; 

 and a central, circular, and more translucent portion, the area 



