CLEAVAGE AND DIFFERENTIATION 129 



a blastomere isolated at the 2-cell stage, the first cleavage which it 

 undergoes after isolation is meridional and equal (corresponding 

 to the second normal cleavage), and its next cleavage is latitudinal 

 and equal (corresponding to the third normal cleavage), and so 

 forth. The first cleavage of a blastomere isolated at the 4-cell stage 

 is latitudinal and equal (corresponding to the third normal cleavage). 

 In other words, the isolated blastomeres cleave as if they were still 

 parts of a whole, but they develop into whole larvae. Here, clearly, 

 the method of cleavage is without effect on the subsequent develop- 

 ment and differentiation. 



The system of cleavage in the sea-urchin egg has been shown to 

 depend on a number of factors. First, there is the control which 

 the cytoplasm exerts on the orientation of the division spindles; 

 this is of such a kind that for a certain period of time (normally 

 corresponding to that between fertilisation and the attainment of 

 the 4-cell stage) any nuclear spindles that there may happen to be 

 are restricted to a latitudinal plane so that division will be meri- 

 dional; after this period, the spindles are rotated into the longitu- 

 dinal axis so that division will be latitudinal. From now onwards 

 there will be two sets of division spindles ; one in the animal and 

 one in the vegetative half of the egg. Those in the former set 

 revert to the latitudinal plane (meridional division of meso- 

 meres), while those in the latter remain longitudinal (latitudinal 

 division of macromeres from micromeres). Experiments of cutting 

 eggs at varying times after fertilisation have shown that the 

 fixation of a division spindle to a given axis is progressively 

 determined: a 1/2 egg cut meridionally within a quarter of an 

 hour of fertilisation can as it were start again with the deter- 

 mination of its spindle axis, and the 1/2 will cleave as a whole egg; 

 a similar 1/2 egg cut meridionally three-quarters of an hour after 

 fertilisation has its spindle axis set and fixed, and it cleaves as 

 a 1/2 blastomere. 



Secondly, there is localised at or near the vegetative pole a special 

 region of cytoplasm which determines a marked inequality of 

 cleavage, leading to the formation of tiny micromeres split off from 

 the large macromeres. Thirdly, there is the fact that this special 

 region of the cytoplasm at the vegetative pole does not acquire its 

 property of causing unequal division until after a certain definite 



HEE 9 



