THE FERTILIZATION OF THE OVUM. 33 



the nuclear spindle predetermines the direction of the cleavage, 

 yet, in this case, the spindle may be formed in any possible 

 direction, and yet the cleavage invariably takes place in the 

 same direction. It is frequently asserted that the division-wall 

 between the daughter-cells appears at right angles to the 

 spindle, and yet, in this case, it may form at a greater or a 

 smaller angle than 90°. The direction of the cleavage is pre- 

 determined not only before the nucleus divides, but long before 

 the asters divide. Certain oscillatory movements of the cyto- 

 plasm, which begin -with the first division of the ovum and can 

 be observed throughout a large part of the cleavage, determine 

 with absolute certainty the direction of the cleavage before any 

 indication of division can be found within the cell. Since the 

 direction of the cleavage is not determined by the nucleus, it 

 follows that the rotation of the cells and the position which 

 they take, with reference to each other, are not determined by 

 the nucleus ; and since it can be shown that the shape of the 

 cleavage cells is very largely the result of intercellular rela- 

 tions, it follows that in this case, at least, the shape of the 

 cell is not determined by the nucleus. 



Still farther, the size of the cell seems to be determined by 

 the cytoplasm rather than by the nucleus. The indirect or 

 karyokenetic division of the nucleus results in an equal division 

 of the chromatic substance. After the division has taken 

 place, the two-daughter nuclei are for a considerable period 

 equal in size, though the cell-bodies in which they lie may be 

 very unequal. Likewise, the division of the asters is always 

 an equal division, and in the early stages of karyokinesis the 

 two asters are always equal in size. Yet in these very stages 

 in which the nuclei and the asters are equal in size, the lobing 

 of the cytoplasm may show beyond doubt that the division of 

 the cell-body is to be very unequal. I believe, therefore, that 

 neither the nuclei nor the asters determine the initial size of 



fertilization (" ISefruchtung," p. 469), mentions the fact that he fertilized ova of 

 one genus of sea-urchin with the sperm of another, from which cross a larval 

 form developed which was intermediate in character l)etween the two genera. 

 The cleavage, however, was purely maternal, thus indicating that it was not 

 influenced by the sperm nucleus. He, therefore, concludes that the process of 

 cleavage, to all appearances at least, is not directed l)y the nucleus. 



