No. 3.] PODARKE OBSCURA VERRILL. 441 



of divisions, but in the shifting of cell areas which occurs 

 throughout the ontogeny. In an elaborate work on Ascaris, 

 Zur Strassen (No. 36) has proposed a theory that attractions 

 (cytotropisms) exist between groups of cells, and that these 

 mutual attractions bring about the shifting of areas, which is 

 a marked feature of the Ascaris embryo. Aside from the fact 

 that there is no positive evidence of the presence of this cyto- 

 tropism, the theory of Zur Strassen is hardly an explanation ; 

 for, supposing that these attractions do exist (and no convincing 

 evidence is given for their presence), their source and nature 

 remain as mysterious as before. The fact cannot be too 

 strongly emphasized that the embryo from the beginning is a 

 complete organism, not a collection of more or less independent 

 units, the " cells," and that controlling the whole development 

 is some force the nature of which we do not know, but which 

 provides that from a given ovum a given organism shall result. 



Equal and Unequal Cleavage. 



It has happened that most of the work on annelid cytogeny 

 previously published has been done on annelids with the 

 unequal type of cleavage, and the development of Podarke, 

 therefore, is of especial interest as affording a means of com- 

 parison between the equal and unequal types. As we have 

 seen, up to the 56-cell stage, all the quadrants of Podarke are 

 alike, the only indication of a differentiation being the constant 

 position of the polar furrow. Since the two ends of this 

 furrow are, so far as we can tell with the microscope, alike, 

 the possibility remains that they are actually alike, and that 

 it is a matter for later determination whether one or the other 

 shall be the posterior end. That this is not the case is, I 

 think, shown by the regular appearance of definite organs at 

 definite times and places in the later development. The small 

 cell, X1.2, appears first as an orientation point, and this is fol- 

 lowed by the bilateral division of 4d and the regular series of 

 events leading to the bilaterally symmetrical embryo. This 

 indicates to my mind that the polarity of the ovum and of the 

 early cleavage stages is as great as in any animal with the 



