284 



THE CHICK. 



cells, which from the first are nucleated, and are arranged in a 

 layer two or three cells deep (Fig. 105, zl). 



In this way, shortly before the time of laying of the egg, the 

 germinal disc becomes converted into a cap of cells, spoken of 

 as tlie blastoderm (Fig. 105). Of these cells the uppermost or 



N Z/\ 





Fig. lOi. —Section through the germinal disc and adjacent parts of the yolk 

 of a Hen's Egg about the middle of its stay in the uteras. The plane of 

 section corresponds to a vertical line drawn through the centre of Fig. 

 103 ; the right-hand end of Fig. 104, which is the future anterior end, cor- 

 responding to the upper border of Fig. 103 ; and the left-hand end of 

 Fig. 104 to the lower or posterior border of Fig. 103. (After Duval.) x 25. 



* N", nucleus of completed segment. K"', nucleus of segment not yet completely separated 



from the yolk. VL, vacuole. Y, yolk. Z A, completed blastomere. ZB, incompletely 

 separated blastomere. 



most superficial layer (Fig. 105, e), which was the first to be 

 definitely established, constitutes th e epiblast_: it consists of a 

 single layer of cells, and is separated by a very shallow space, 

 the blastoccel or se^-mentation caTitv, B, which appears in section 



Fig. 103. — Vertical section of the blastoderm and adjacent part of the yolk 

 of a Hen's Egg towards the close of segmentation. The anterior edge is 

 to the right, the posterior edge to the left hand. (After Duval.) x 25. 



B, blastoccel or segmentation cavity. E, epiblast. W, nucleus of blastomere, wbicli 

 as yet is only incompletely separated from the yolk. VL, vacuole. Y jyolk. ZL, one 

 of the lower-layer cells or blastomeres. 



as a mere chink or split, from the deeper mass of cells which 

 may be spoken of collectively as lower layer celle^ zl. 



During the rest of the time that the egg stays in the uterus, 

 while the egg-shell is forming, the process of segmentation 

 continues actively. The clefts extend to the edge of the germinal 

 disc, which becomes shai'i^ly marked off from the yolk beyond it ; 



