SEGMENTATION (CLEAVAGE) OF THE EGG. 



469 



followed by others, some of which are again radial in arrange- 

 ment, whilst others join up two or more radial furrows. In 

 this way the surface of the disc is, as it were, converted into 



Fig. 77. An early stage in the segmentation of the germinal disc of 

 the Hen's Egg. (After Coste and Duval.) x 10. 



Fig. 78. A later stage, in which the germinal disc has increased in 

 size and the segments have, by further division, become smaller 

 and more numerous. (After Coste and Duval.) x 10. 



Both these figures are from eggs taken from the lower part of 

 the oviduct of the hen. 



a mosaic of small polygonal areas, bounded peripherally by 

 larger irregular segments which form the germ- wall.' 



Fig. 79. Section through the germinal disc and adjacent parts of the 

 yolk of a Hen's Egg about the middle of its stay in uterus. The 

 plane of section corresponds to a vertical line drawn through the 

 centre of Fig. 78. (After Duval.) x 25. 



N, nucleus of completed segment. N', nucleus of segment not yet completely 

 separated from the yolk. VL, vacuole. Y, yolk. ZA, completed blastomere. 

 ZB, incompletely separated blastomere. 



This areolation is an expression of cell-division. The 

 zygote-nucleus divides before the cleavage of cytoplasm takes 



