CLEAVAGE AND DIFFERENTIATION 133 



But at the start of cleavage, most of this nucleic acid is in the 

 cytoplasm. At each cleavage division, the nuclei of the daughter- 

 blastomeres are slightly larger than half the nucleus of the blasto- 

 mere that gave rise to them. There is consequently a gradual return 

 of nuclear material from the cytoplasm into the nuclei of the blasto- 

 meres, and this is shown by the drop in the ratio of total volume of 

 cytoplasm to total volume of nuclei at successive stages of cleavage. 

 At the 4-cell stage the ratio is about i8 : i, at the 64-cell stage it is 

 12 : I, while in the blastula the ratio has returned to the original 

 value of 7 : i.^ 



These results are of considerable interest, and for two reasons. 

 In the first place, the return of the cytoplasmo-nuclear ratio to the 

 original value occurs in the blastula, when cleavage has ended, and 

 when the hereditary effects of the nuclear material can begin to 

 manifest themselves, as will be shown in Chap. xii. It is not im- 

 probable that these two sets of events are causally related. In the 

 second place, the recognition of the existence in the cytoplasm of 

 the ripe egg of a finite amount of nuclear material accounts for the 

 termination of cleavage. It is well known that eggs which are made 

 to develop in the haploid condition (as by artificial partheno- 

 genesis) go on cleaving until their cells are half the volume of 

 normal diploid cells. ^ The haploid nuclei of the blastomeres re- 

 quiring only half the amount of nuclear material from the cyto- 

 plasm, the supply in the cytoplasm will last longer than is the case 

 with diploid nuclei ; cleavage will therefore go on for a longer time, 

 and the cells will be smaller. Conversely, it is known that if half 

 an egg, containing a nucleus, is fertilised (that is to say, diploid 

 nuclei but only half the normal quantity of cytoplasm is present), 

 the resulting larva has cells of normal (diploid) volume but is itself 

 of half size. It follows that it has half the number of cells that the 

 normal has, and this is what would be expected since it had only 

 half the reserves of nuclear material in the cytoplasm. Lastly, it is 

 possible in some cases to obtain fertilised eggs with tetraploid 

 nuclei. The size of the embryos which these produce is normal, but 

 their cells are twice as large and half as numerous as normal. The 

 quantity of nuclear reserve materials in the cytoplasm has given 

 out sooner than during normal cleavage, with the result that the 

 division of the blastomeres has not proceeded so far. 



^ Godlevvski, 1925. " Boveri, 1905. 



