94 



SCIENCE 



[N. S. Vol. XXVII. No. 681 



stances to the cell at every mitosis, thus 

 constituting a sort of systole and diastole 

 of the nucleus, and it may be suggested 

 that this interchange between nvicleiis and 

 cytoplasm is one of the primary func- 

 tions of mitosis. Many of the substances 

 which are liberated from the nucleus are 

 visibly different from the other substances 

 of the cell body, and in some cases they 

 may be traced through successive stages of 

 development until they give rise to par- 

 ticular portions of the embryo. 



The time at which particular embryonic 

 differentiations appear differs widely in 

 different animals. In some cases the earli- 

 est visible differentiations which may be 

 correlated with the later differentiations of 

 the embryo or larva, appear about the time 

 of the formation of the blastula or 

 gastrula, and here the cleavage cells are 

 apparently all alike. In other cases the 

 cleavage cells are not alike; even the first 

 cleavage, as well as the later ones, may be 

 of differential value, thus giving rise to 

 cells which differ in size, symmetry and 

 substance, and these features of the cleav- 

 age are inherited as certainly as are the 

 form and character of the larva or adult. 

 In still other cases marked differentiations 

 of the egg are visible before cleavage be- 

 gins, while in some instances these differ- 

 entiations are present even while the eggs 

 are still in the ovary. 



But whether these differentiations ap- 

 pear early or late, there is reason to believe 

 that the processes by which they arise are 

 essentially the same in all cases, and that 

 there is therefore no fundamental differ- 

 ence between eggs which differentiate early 

 and those which differentiate late. It is 

 quite possible that the time of appearance 

 of differentiations depends less upon the 

 time of formation of different substances 

 in the egg than upon the time of their local- 

 ization or segregation in specific regions or 

 cells. All eggs hitherto examined show 



certain differentiations of the cytoplasm 

 even in the earliest stages, though in most 

 eases the different substances are not segre- 

 gated before the cleavage stages. When 

 such eggs are submitted to a strong cen^ 

 trifugal force the substances of the unseg- 

 mented eggs of practically all animals may 

 be separated into at least three zones, which 

 correspond in a striking manner to the 

 three zones which are normally present in 

 the unsegmented eggs of ascidians. This 

 indicates that the real difference between 

 eggs which are highly differentiated and 

 those which show little or no differentiation 

 of the cytoplasm may be in the segregation 

 of unlike substances rather than in the 

 presence or absence of such substances. 



Regarding the other factor of differen- 

 tiation, viz., the segregation or localization 

 of unlike substances, rather more is known 

 than in the case of their origin. Segrega- 

 tion of the different substances of the un- 

 segmented egg and of the cleavage cells is 

 known to take place chiefly by protoplasmic 

 flowing, the direction of the flow being cor- 

 related with the initial polarity of the cell, 

 and with the chemotropism of the sub- 

 stances concerned. Segregation thus pro- 

 duced is still further emphasized and ren- 

 dered permanent by cell division and the 

 formation of partition walls. I have re- 

 cently found that when different substances 

 of the egg are displaced by strong centri- 

 fuging they tend to come back to their nor- 

 mal positions unless prevented by partition 

 walls which have formed in the meantime. 

 In the early cleavage stages of many ani- 

 mals the cell divisions are differential both 

 as regards the size and the substance of the 

 daughter cells, but in such cases the cleavage 

 is not responsible for the differentiation as 

 is plainly shown by the fact that the segre- 

 gation of the different substances occurs 

 before division, and the inequality of the 

 cleavage may be foreshadowed by lobes 

 of the cytoplasm even before the nucleus 



