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 
molluscan 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 surrounding it, these variations leading, on the one 
hand, to the formation of the molluscan cross: on the other, to the 
annelidan. 
Second Quartet. 
While 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 leotropic direction, 
the upper four giving off the four tip cells (2a""-2d") toward the upper 
pole, while the lower four give origin to small cells resembling the 
