CLEAVAGE AND DIFFERENTIATION 131 



period of time. In normal cleavage, this time corresponds to the 

 attainment of the 8-cell stage. 



By various methods (use of dilute sea- water, shaking, and cutting 

 the egg into halves), it is possible to alter the time-relations of 

 mitosis relatively to these three factors. By delaying the rate of 

 cell-division, it is possible to make the second, or even the first 

 cleavage of an egg fall into the period when the nuclear spindles are 

 forced into the longitudinal axis. The result will be latitudinal 

 division at the 2-cell and i-cell stages respectively, whereas it 

 normally happens at the 4-cell stage. Very instructive are the cases 

 in which the cleavage division falls during the change of position of 

 the nuclear spindles, i.e. when the latter are oblique. One more 

 cleavage division in eggs whose mitoses are thus delayed w^U lead 

 to formation of micromeres precociously (fig. 61). 



It will thus be seen that it is possible to make a whole egg cleave 

 as if it were an isolated blastomere of the 2-cell or 4-cell stage. When 

 a blastomere is isolated from a normal egg, the mitotic speed of 

 which has not been interfered with, the subsequent cleavage 

 divisions continue to be governed by the same factors as in the 

 normal tgg, with the result, therefore, that the blastomere cleaves 

 as a part.i 



The second example of the relative unimportance of cleavage as 

 regards differentiation is provided by those cases in which a frog's 

 egg has been penetrated by several sperms. One sperm-nucleus 

 fuses with the egg-nucleus, but the other sperm-nuclei remain 

 isolated in the cytoplasm of the egg. When the egg begins to under- 

 go cleavage, not only does the zygote-nucleus divide and induce the 

 division of the cytoplasm into blastomeres, but each of the isolated 

 sperm-nuclei has a portion of cytoplasm allotted to it, and this be- 

 comes separated off as a little blastomere and subsequently divides. 

 Cleavage is therefore very irregular, and the embryo is composed 

 of an indiscriminate mixture of blastomeres, some containing the 

 products of division of the zygote-nucleus and representing the 

 normal blastomeres of typical cleavage, and some representing 

 blastomeres which would normally never have come into existence. 

 The two kinds of blastomeres can be recognised without difficulty, 

 for those derived from the zygote-nucleus are of course diploid, 



^ Driesch, igoo; Horstadius, 1928. 



9-2 



