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



443 



although the two crosses have exactly the same structure, they 

 have a completely different origin. Are they to be regarded as 

 homologous or not } The different origin of the cells does not 

 in itself necessarily gave a negative answer to this question. 

 Aside from this, however, there is good reason for believing 

 that they are not homologous. Conklin and Blochmann agree 

 that, in the molluscan cross, two of the arms fall ultimately in 

 the median line, while the other two are transverse to it. The 

 annelidan cross, on the other hand, is inclined at an angle of 

 45° to the median and transverse planes. They must, therefore, 

 give rise in the two cases to different regions of the upper hemi- 

 sphere. Taking this in connection with the different history of 

 the cells, there is every reason to believe that the annelidan 

 and the molluscan crosses are analogous, but not homologous, 

 structures, whose origin is in some way connected with the 

 mechanical conditions of cleavage. What these conditions are 

 I am unable to conjecture. 



We come in the last place to the relation of the molluscan 

 velum to the annelidan prototroch. According to Blochmann 

 the velum arises from the terminal cells of the lateral arms of 

 the cross (" Ur-velarzellen ") which travel forwards, give rise to 

 a transverse series of cells on either side, and from these arise 

 the corresponding halves of the velum. If this account be cor- 

 rect, the velum has a totally different origin from the annelidan 

 prototroch (in the annelid from a)-'^, b^-^, c^-'^, d'^-^ ; in the mollusk 

 from fl2.2.i^ ^2.2.i^_ Does this indicate that the two structures are 

 not homologous .? I do not think that this conclusion neces- 

 sarily follows. 



X. General Interpretation of the Cleavage. 



It is impossible to reflect upon the complicated yet perfectly 

 ordered events of the cleavage in Nereis without attempting to 

 discover the nature of the causes by which their course is deter- 

 mined. The completeness with which the history of the indi- 

 vidual blastomeres can be followed and their mutual relations 

 determined, the definite periods into which the ontogeny falls, 

 the sudden transitions from one phase of activity to another — 

 these and many other features of the development bring to 

 the foreground some of the most interesting and fundamental 

 problems of cytology. The solution of many, perhaps all, of 



