66 CLEAVAGE II 



qualitatively different, then the distribution of the determinants 

 contained in them would be abnormal, and should cause an 

 abnormal differentiation of the cytoplasm. As a matter of 

 fact the egg develops into a normal tadpole. 



The theory of the qualitative division of the nucleus is 

 therefore proved untenable from more than one side. It is 

 only fair to Roux to state that he has some time since 

 abandoned an impossible position. 



It only remains for us to review briefly the evidence of 

 a like nature drawn from experiments on the eggs of other 

 forms. 



Experiments similar to that just quoted have shown that 

 the pattern of cleavage may be altered by pressure without 

 interfering with the normality of development in, for in- 

 stance, the eggs of the Sea-urchin (Driesch) and of the worm 

 Nereis (Wilson). In Nereis the egg segments into a flat 

 plate of eight cells. On releasing the pressure, an octette 

 of micromeres is formed instead of the usual quartette, and 

 a normal trochophore is developed. The pattern of cleavage 

 may be altered by other means. Thus in Cerebratulus (a 

 Nemertine) in calcium-free sea-water the third division is 

 meridional, then a first, and later a second, octette of micro- 

 meres are formed. Again, in certain cases the egg divides 

 simultaneously into three, a meridional division produces 

 six cells in a ring, sextettes of micromeres are then given 

 off. In both cases a normal Pilidium is produced (Yatsu). 

 In Sea-urchin eggs the blastomeres may be deranged by heat, 

 shaking, and diluting the sea-water to any extent or almost 

 so, without prejudice to the eventual normality of develop- 

 ment. 



In artificial parthenogenesis again, an irregular segmenta- 

 tion may be followed by a regular development. 



Secondly, isolated blastomeres frequently segment as parts, 

 that is, as though the remaining blastomeres were present. 

 Thus a half-blastomere of Echinus gives rise to four meso- 

 meres (animal cells), two macromeres, and two micromeres, 

 and a quarter-blastomere to two mesomeres, one macromere, 

 and one micromere. The isolated blastomeres of spirally 



