1904.] NATURAL SCIENCES OF PHILADELPHIA. 351 



Annelids Conklin says: "The cross and rosette series are the direct 

 result of the position, size and shape of their constituent cells". The 

 original position of cells resulting from regularly alternating spiral 

 cleavages is a function of that mode of division. The shape of cells 

 depends largely upon the relations which they bear to one another. 

 Their size is not so easily explained, and upon this factor depends, to a 

 large extent, the varying forms of crosses met with in different in- 

 stances. If it be supposed that the original arrangement of the upper 

 pole cells of Mollusk and Annelid eggs was radial in form, the modifi- 

 cations which have arisen in the two groups may, in part at least, be 

 referred directly to the size of the cells comprising that area. The 

 importance and early development of the trochoblasts of Annelids 

 has resulted in encroachment upon that area which in the segmenting 

 eggs of these forms corresponds to the cross region of Mollusks. As a 

 result the ''intermediate" series of Annelids, corresponding to the 

 moUuscan cross cells, lack the prominence characteristic of the same 

 cells in the latter group. ^Moreover, it is interesting to note that such 

 a Mollusk as Ischnochiton, which in the development of its trocho- 

 blasts and prototroch shows a condition intermediate between Mol- 

 lusks and Annelids, also exhibits a cross which is intermediate in 

 character. Though the trochoblasts have been taken here as an ex- 

 ample of the influence which variation in size or rate of division may 

 have upon the primitive arrangement of blastomeres in the spirally 

 cleaving egg, it is doubtless true that other cells may in like manner 

 undergo modifications which will result in similar rearrangements. 



Thus it may be concluded that the group of cells constituting the 

 cross owes its radial arrangement primarily to the form of cleavages 

 by which it arose, but that the cross as a definitely marked structure 

 is the result of variations in the size, shape and rate of division of the 

 cells comprising or suiTOunding it, these variations leading, on the one 

 hand, to the formation of the moUuscan cross:' on the other, to the 

 annelid an. 



Second Quartet. 



Wliile the egg is yet radially symmetrical and its blastomeres num- 

 ber twenty-four, the original second quartet cell of each quadrant has 

 divided in a dexiotropic direction into cells of equal size. After the 

 mesentoblast has arisen, but before the basal cells of the cross are 

 formed, all of the second quartet cells divide in a Inotropic direction, 

 the upper four giving off the four tip cells (2a"-2d") toward the upper 

 Dole, while the lower four give origin to small cells resembling the 



