74 DIFFERENTIATION III 



the larva reared from the lobeless D cell has an apical organ 

 though still devoid of a trunk. The polar lobe of the egg, 

 therefore, contains some factor on which the development 

 of the apical organ, as well as that of the trunk, depends, 

 and the factor for the sense-organ moves from its original 

 position in the vegetative hemisphere to its definitive posi- 

 tion at the animal pole between the first and the second 

 divisions. 



In Ascaris again where the cells from which certain organs 

 come can be distinguished from the beginning the isolated 

 blastomeres have only a partial development (Stevens). Thus 

 S 1 (Fig. 5, p. 13) gives rise to an ectoblastic vesicle, P l to P 2 

 and EM St and the derivatives of these, and so on. Removal 

 of one ectodermal cell (A) in the four-celled stage does not, 

 however, very seriously interfere with the development of the 

 remaining three into a normal embryo. Strictly speaking, 

 the blastomeres are not isolated, but one or more is killed by 

 ultra-violet light. 



If, however, the egg be centrifuged it divides meridionally, 

 and each cell then segments like a whole egg, with diminu- 

 tion of the chromosomes in the somatic cells. Lastly, in 

 Ascidians the cytoplasm has a very definite structure (easily 

 visible in Cynthia, only with the help of reagents in Phallusia), 

 the parts of which assume their definitive bilateral arrange- 

 ment as a result of fertilization. This bilaterality persists as 

 that of the embryo, and since the first furrow (meridional) 

 coincides with the plane of symmetry and the sagittal plane, 

 and the subsequent furrows (meridional, latitudinal, and so on) 

 have very definite positions with regard to the first, the 

 various regions of the cytoplasm necessarily pass into particu- 

 lar cells. It is not therefore surprising that a cell should never 

 be able, when the others have been killed, to give rise to any 

 more than it would have done in the whole egg. As in 

 Ascaris and in the Mollusca the potentialities of isolated cells 

 are restricted ab initio. 



While the cytology of the ovum shows us that its cytoplasm 

 has always a certain structure, though different in different 

 cases, a structure which assumes its definitive form during 



