CELL-DIVISION 



across the e^g in two parallel rows, one on each side of the 

 cleavage-furrow. 



Concurrently with these nuclear and cytoplasmic events, 

 the clear peripheral cytoplasm, the hyaline plasma-layer, 

 exhibits changes. During the progress of the sperm- 

 head toward the egg-centre, the hyaline plasma-layer 

 is everywhere of equal width; but with the beginning of 

 the cleavage-process it appears delicately crenated, irregular 

 in contour. The egg at this time being somewhat darker 

 because of closer approximation of the yolk-spheres gives 

 the picture of cytoplasmic contraction, a condition which 

 passes leaving the egg clearer and the hyaline plasma-layer 

 of smoother contour. Immediately prior to the egg's 

 elongation, the hyaline plasma-layer is thinner over the 

 poles of the long axis of the transparent dumb-bell shaped 

 area and seems slightly thicker at the equator, the site of 

 the future cleavage-furrow.^ With cleavage this thickening 

 apparently increases; actually, the filaments of the hyaline 

 plasma-layer are stretched, because the plasma-membrane 

 to which they are attached does not move inward with the 

 cleavage furrow; they are therefore longer here than 

 elsewhere. 



We can confirm and extend these observations on the 

 living egg by study of properly fixed normal eggs. The 

 accompanying figures are from sections of fixed Arhacia 



eggs. 



Fig. 33«2 is of an egg with intact nucleus, centrospheres 

 and asters. Fig. 33^ shows an egg with ruptured nucleus 

 and emerging chromosomes (seen in the living egg as hya- 

 line droplets); here the centrospheres are larger and the 

 astral rays more extended. As the heavily stained chro- 

 mosomes move apart, the centrospheres and asters increase 

 in size (Figs. 33r, 33^) finally to reach the maximum 



1 Cf. Ziegler^ 1^04. 



257 



