ADAPTATION IN CLEAVAGE. 59 



to a wave of condensation in the cytoplasm, travelling outwards, 

 sweeping away the yolk-granules, but leaving the cytoplasm 

 behind. The latter theory would explain the appearance of 

 vesicular cytoplasm within the boundary during the phenome- 

 non, and so avoid the difficulty of explaining the extremely 

 rapid growth of substance within the sphere ; it could be com- 

 pared to the wave that spreads out from a pebble dropped in 

 the water. But it is difficult to understand the nature of such 

 a disturbance. On the whole the expansion theory seems to 

 me much more probable than the wave theory, and the latter is 

 certainly untenable in the first slow stages of the process. 



Another important thing to notice is that just before the 

 expansion begins the sphere is three-quarters surrounded by 

 the chromosomes, and I would like to hazard the conjecture 

 that, at this time, there may be a diffusion of some chromatin 

 substance within the sphere, the interior of which tends to 

 stain more darkly than before. If this be actually the case, it 

 has an extremely interesting bearing on subsequent events. 



Let us now trace the further actions of the germ-nuclei and 

 sphere-substance during the first cleavage. In general, what I 

 propose to show is that the sphere-substance moves and elon- 

 gates so as to mark out a definite horizontal plane in the ^g^, 

 and that the first cleavage-spindle places itself in conformity 

 with this predetermined arrangement. 



Each of the germ-nuclei, at the close of the maturation, is a 

 small, dense clump of chromosomes, and both begin to swell up 

 into the vesicular form of the ordinary resting nucleus at the 

 same time, and keep step throughout the process. This indi- 

 cates a new general condition of the egg-cytoplasm ; for, while 

 we might explain the enlargement of the egg-nucleus alone as 

 part of the usual sequence of mitosis, due to purely localized 

 conditions of the egg-substance, we can only explain the effect 

 simultaneously produced on both nuclei by the assumption 

 that the entire egg-cytoplasm is entering on a new phase of its 

 development. 



The first movements of the germ-nuclei begin after each has 

 enlarged considerably. In its early movements the egg-nucleus 

 is preceded by the sphere-substance, which moves towards the 



