234 



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



ZB 



\ 



FIG. 104. Section through the germinal disc and adjacent parts of the yolk 

 of a Hen's Egg about the middle of its stay in the uterus. 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 2f>. 



N, nucleus of completed segment. W, nucleus of segment not yet completely separated 

 from the yolk. VL, vaenole. Y, yolk. ZA, completed blastomere. ZB. incompletely 

 separated blastoinere. 



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

 definitely established, constitutes the epiblast ; it consists of a 

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

 the blastoccel or segmentation cavity. B, which appears in section 



FIG. 105. 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, epi blast. W, nucleus of blastomere, which 

 as yet is only incompletely separated from the yolk. VL, vacuole. Y .yolk. 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 cells, 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 sharply marked off from the yolk beyond it ; 



