262 WATASE. [Vol. IV. 



the outline of the original mother-cells. He points out that the 

 mode of cytoplasmic cleavage of cells depends chiefly upon the 

 configuration of a growing organ, and I may add, of an organ- 

 ism, at the time when it begins to grow, and not upon its phy- 

 letic significance. As an example, he cites the cleavage of 

 cells in a glandular hair of the gourd-plant and the embryo of a 

 Phanerogam, both of which undergo essentially the same "type" 

 of cleavage in every detail so far as we can judge from the re- 

 semblances in the external appearances of their cleavage furrows. 



Balfour pointed out long ago that the similarity or dissimi- 

 larity of the cleavage process in the ova of different animals as 

 indicated by the external phenomena alone, has no value what- 

 ever in estimating the systematic affinities of different organisms 

 which develop from them. Given two animal ova with their 

 external configurations alike, owing to the similar distribution 

 of food-yolk and of the germ-protoplasm, then the early phe- 

 nomena of cleavage will be also alike, whatever may be the 

 difference of animals which later develop from them. 



That the argument based on the arrangement of superficial 

 furrows alone is not entitled to any weight, is further shown by 

 their total absence in several forms of ova, which nevertheless 

 develop into perfect organisms. It has been shown that in a 

 certain plant, the cytoplasm becomes divided without a corre- 

 sponding division of its nucleus.^ Such facts seem to point to 

 the conclusion that the division of the cytoplasm and that of 

 the nucleus are two independent phenomena, and that one pro- 

 cess can occur without the other, and that when they do occur 

 in close succession, as in ordinary cell-division, it is to be looked 

 upon as a case of coincidence. ^ At any rate, the following con- 

 clusion seems to be a valid one ; viz. that the division of the 

 nucleus and that of the cytoplasm are due to different causes. 

 The cleavage furrows are devoid of significance, taken by 

 themselves. The formation of a cleavage furrow is a negative 

 phenomenon, a reflex manifestation of some other activities 

 going on elsewhere. 



It is now quite generally conceded that the nucleus of the 



^ For different examples in which the caryokinetic division of the nucleus is not 

 followed by the division of the cytoplasm, see Kolliker : Handbuck der Gewebelehre. 

 6th edition, p. 6i, 1889. 



2 Sachs: Lectures on the Physiology of Plants. 1 887. 



