444 WILSON. [Vol. VI. 



these problems may lie far in the future. Yet an attempt to 

 consider them may, at least, serve one useful purpose by defin- 

 ing more clearly their outlines ; and it is possible that the 

 embryo of Nereis may prove in some respects an unusually fav- 

 orable object for their investigation. 



What is the significance of the spiral and bilateral forms of 

 cleavage, and where lie the causes that determine the transfor- 

 mation of the one into the other t What determines the form 

 and succession of the divisions of the individual blastomeres, 

 which, as in the case of the first somatoblast, may have so com- 

 plicated and yet so definite a history .? Is the blastomere, like 

 the ovum, a self-regulating mechanism that contains within itself 

 the causes of its own transformations, that is wound up like a 

 clock, as it were, and must of necessity run the course prede- 

 termined in its own structure } Or are its successive phases 

 of activity determined or guided by influences proceeding from 

 without — by the interaction of the cell with its fellows in the 

 cell-complex } I scarcely need to point out that this latter 

 question bears immediately upon that most pressing of all cur- 

 rent general questions, the relation between the somatic cells 

 and the germ-cells ; and it is a problem upon which a flood of 

 light is being thrown by recent investigations. It must be said 

 at the outset that the development of Nereis gives no certain 

 answer to this question. It does, however, as I beheve, give 

 strong indirect evidence which is worthy of attentive con- 

 sideration. 



The most striking feature in the cleavage, and the one on 

 which the entire discussion may be made to turn, is the sudden 

 appearance of bilateral symmetry in the cleavage. The meaning 

 of the bilateral cleavages in themselves is perfectly obvious. 

 They are the forerunners of the bilateral arrangement of parts 

 in the adult ; and, as such, their explanation belongs to the 

 general problem of bilateral symmetry, which need not be con- 

 sidered here. The all-important point is that the bilaterality 

 does not appear at the beginning of development. It appears 

 only at a comparatively late stage, and by a change so abrupt 

 and striking as to possess an absolutely dramatic interest. I 

 lay especial stress on this point because it is wholly opposed 

 to the opinion, which has of late rapidly gained ground among 

 embryologists, that (i) the bilaterality of the adult is predeter- 



