No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 267 



The result of these divisions is a 64-cell stage, which is like 

 that of Amphitrite and Clymenella, except that it is more 

 regular ; so regular, in fact, that there is no way of distinguish- 

 ing one quadrant from another. It is easy to distinguish the 

 two poles and thus the Qgg axis, but not the sagittal plane of 

 the embryo. One can distinguish from one another the com- 

 ponent cells of a quadrant, but cannot say to which quadrant 

 they belong. This is true throughout the history of the cleav- 

 age, as far as I have studied it. 



The apical rosette is formed just as m Amphitrite, Clymenella, 

 etc., though the origin of the cells is peculiar in that they lie 

 at first in the segmentation cavity but later elongate and reach 

 the surface (Fig. 98). Since the polar globules may be in any 

 of the cross cells, they are sometimes found far removed from 

 their original position (p. 9, Figs. 98, 100, lOi, 104). 



At the vegetative pole the eight lower cells, A^, B^, C^, D^ 

 and a", b*, c^, d^, form what may be called provisionally the 

 entoderm plate. These do not divide again at the surface, 

 but become elongated, and their nuclei migrate inward like 

 those of the mesoderm and entoderm cells of Amphitrite (Figs. 

 102, 103). 



It is interesting to notice the contrast between the divisions 

 of the animal and the vegetative-pole cells in this last cleavage. 

 The two cell-groups in question were of about the same size 

 and similarly arranged, four cells at the animal pole and four at 

 the vegetative pole. All the cells of both groups divide in the 

 same direction and at the same time, but at the animal pole 

 the result is eight cells, of which the four central ones are the 

 smaller, while at the vegetative pole the result is exactly the 

 reverse — the four central cells are the larger. The relative 

 position of the cells at the two poles of the egg can be seen in 



Fig- 95- 



At the 64-cell stage the living Q.gg shows distinctly the 

 rosette, undivided cross cells, and the prototrochal cilia borne, 

 in part at least, by the cells corresponding to the primary 

 prototroch in Amphitrite (Fig. 97, shaded cells). 



The situation of the fully formed prototroch and the size 

 and position of its component cells sustain the interpretation 



