132 CLEAVAGE AND DIFFERENTIATION 



while the others are haploid. Since the volume of the cell is pro- 

 portional to the quantity of nuclear material which it contains, it is 

 easy to recognise the descendants of the two kinds of blastomeres in 

 the tissues to which they give rise. In spite of their abnormal 

 cleavage, such polyspermic frogs' eggs can sometimes develop 

 normally, the stage ultimately reached depending on the number 

 of supernumerary sperms present. A pentaspermic egg can produce 

 a free-swimming tadpole which lives for 10 days after hatching:^ a 

 dispermic egg can produce a tadpole which lives for three months.^ 

 Lastly, it has been shown in the case of Chaetopterus and Nereis 

 that a certain amount of differentiation can take place even if cleav- 

 age is totally suppressed, by treatment of the egg with KCl.^ Cilia 

 are put out and internal rearrangements occur, the most interesting 

 of which is the assumption by certain granules of the position in 

 the Qgg which corresponds to that of the cells of the prototroch, 

 which cells in normal cleavage come to contain these granules.'* 



§8 



But besides splitting up the cytoplasm of the egg into smaller units, 

 cleavage has one very important effect, though its bearing on differ- 

 entiation is indirect, and this concerns the adjustment of the ratio 

 between amount of nuclear matter and amount of cytoplasm present 

 in the cell. 



In the oocyte of the sea-urchin {Echinus niicrotuberciilatus) it has 

 been shown that the ratio between the volume of the cytoplasm 

 and that of the nucleus is 7 : i . Maturation results in a certain in- 

 crease in cytoplasmic volume and a reduction in nuclear volume, 

 so that the ratio of cytoplasm to nucleus in the ripe egg is 400 : i . 

 But the volume of the cytoplasm has been only about doubled, so 

 that the explanation of the high ratio in the ripe egg must be looked 

 for to a small extent in the extrusion of nuclear material in the polar 

 bodies, and to a large extent in the passage of nuclear material into 

 the cytoplasm. Now the total amount of nucleic acid in the egg 

 and in subsequent stages of cleavage up to the blastula is constant.^ 



^ Brachet, 1910. - Herlant, 191 1. ^ Lillie, 1902; Spek, 1930, 



^ What is in some ways a complementary experiment has been carried out by 

 removing the zygote nucleus from uncleaved axolotl eggs by means of a micro- 

 pipette. In spite of the absence of nuclei, the cytoplasm makes an attempt to 

 carry out cleavage, though this is partial and irregular. Jollos and Peterfi, 1923. 

 ^ Masing, 1910. 



