No. 3-] THE CELL-LINEAGE OF NEREIS. 441 



the central (primary) micromeres and the outer belt. Finally, 

 the second set of micromeres divide meridionally, and in Nereis 

 and Crepidiila this can be distinctly recognized as a left-handed 

 spiral. Thus arise four pairs of cells, with which alternate the 

 four micromeres of the third set ; and the outer belt now con- 

 sists of twelve cells, which form the lips of the blastopore 

 (C, D,E). 



All three agree, furthermore, in the meridional division of the 

 first-formed progeny of the primary micromeres {p)--^, b^-^, c^-^, 

 d^-^), so that the inner belt is likewise composed of twelve cells. 

 The third spiral cleavage of the primary micromeres gives rise 

 to four apical cells (the "rosette-cells " of Nereis, the " Scheitel- 

 zellen " of Eitrylepta and Discoccelis), which strikingly recall the 

 four apical cells of the Echimis embryo (Selenke, No. 23). At 

 this point the polyclade diverges from the annelid and mollusk. 

 In the former, the differentiation of the germ-layers is complete. 

 In the latter two, one further step is taken, by the separation of 

 the primary mesoblast-cell from the left posterior macromere. 



The general later history of the blastomeres thus formed is as 

 follows : In the polyclade tJie first groitp of micromeres gives rise 

 to the entire ectoblast, the second and third groups to the meso- 

 blast, the macromeres to the entoblast. In the mollusk and anne- 

 lid, on the other hand, the second and third groips of micromeres 

 give rise to ectoblast, like the first set, and the mesoblast arises 

 subsequently. This remarkable divergence between the polyclade 

 on the one hand and the mollusk and annelid on the other is a 

 fact of capital importance, for it proves that cells having pre- 

 cisely the same origin in the cleavage, occupying the same posi- 

 tion in the embryo, and placed under the same mechanical 

 conditions, may nevertheless differ fundamentally in morpholog- 

 ical significance. We cannot escape the conclusion that the 

 cell possesses a definite hereditary tendency upon which pri- 

 marily its nature depends, however much its outward form or 

 mode of division may be affected by the mechanical conditions 

 of its environment in the body ; and full weight must be given 

 to this heredity in every attempt to interpret the origin and 

 meaning of cleavage-forms. 



Let us now pursue the comparison between the annelid and 

 the mollusk. Blochmann describes and figures on the upper 

 hemisphere of the Neritina embryo a perfectly symmetrical cross 



