No. 3.] THE CELL-LINEAGE OF NEREIS. 445 



mined in the ovum and therefore (2) manifests itself through- 

 out the cleavage. The first of these propositions is a barren 

 truism, for every adult characteristic is in one sense predeter- 

 mined in the ovum. The second, however, is a premature 

 generalization, and it does not follow from the first. /;/ some 

 cases, it is true, the cleavage does show a bilaterality from the 

 beginning ; and sometimes it is marvellously perfect, as shown, 

 for instance, by the brilliant studies of van Beneden and Julin 

 on the ascidian cleavage (No. 24) and of Watase on that of 

 the cephalopod (No. 26). But this is by no means invariably 

 the case. The earlier stages of many annelids, mollusks, poly- 

 clades, and other bilateral forms are bilateral neither in structure 

 nor in the form of cell-division. It is true that the adult bilat- 

 erality may be in a certain way foreshadowed in the spiral 

 period {cf. p. 400). But the following facts are indisputable : 

 (i) that at no time during the spiral period is it possible to 

 pass a plane so as to divide the body into equivalent right and 

 left halves ; and (2) that the planes of cleavage throughout this 

 period are symmetrically distributed with respect, not to the 

 future median plane, but to the vertical axis of the ovum. 



What determines the period at which the first bilateral divis- 

 ions appear.'' To this question a very simple and obvious 

 answer can be given which, as I believe, gives a key to the 

 entire development. The bilateral asymmetry of the early stages 

 depends mainly upon the fact that the substance of the somato- 

 blasts (i.e. the ^nesoblast and the material of the ventral plate) is 

 stored in the left posterior macromere. Bilateral symmetry is 

 establisJied upon the reduction of this macromere {D) to the size 

 of its fellow {C) by the separation of the somatoblasts and their 

 transportation to the median line. Immediately tipon this event 

 follows the appearance of bilateral cleavages throughout the em- 

 bryo, except in the cells which give rise to the prototroch, a purely 

 larval organ. In other words, the bilateral cleavages appear as 

 soon as the embryonic material becomes equally distributed with 

 respect to a plane, which ultimately becomes the adult plane of 

 symmetry. Is the correspondence between these two events 

 a mere coincidence, or do they stand in the relation of cause 

 and effect } Under the former alternative we must suppose 

 that the coincidence is the result of a kind of pre-established 

 harmony, such that the cells of the upper and lower hemi- 



