No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 299 



there is a good deal of symmetry in the formation of this 

 organ, since each quadrant contributes exactly the same num- 

 ber of cells and in the same manner, except that the dorsal 

 quadrant furnishes no secondary prototrochal cells. 



On the other hand, in AmphiUite the determinants of the 

 paratroch — a strictly bilateral organ — are all within one cell, 

 the somatoblast, of the i6-cell stage, and at the next division 

 the determinants of three of the four cells of the paratroch 

 come to lie in one of the resulting cells, and those of the fourth 

 in the other. Though the succeeding cleavages of these two 

 somatoblast cells are quite different, nevertheless, after several 

 divisions, the determinants come together again in control of 

 four cells, which are bilaterally placed with respect to the larva 

 and belong to the same generation, — the eleventh, counting 

 the ovum as the first. That this is significant is indicated by 

 the fact that two extremely minute cells are " budded off " just 

 before the complete differentiation of the paratroch. Otherwise, 

 of course, two of the cells would belong to the tenth generation. 



When the origin of the prototroch and of the paratroch 

 has been ascertained in a larger number of forms, we shall 

 be in a position to discuss whether the destiny of cells is 

 conditioned upon their contained determinants or upon their 

 position. 



III. Axial Relations. 



The discussion of the axial relations of the egg of Amphitrite 

 naturally falls into two chapters : the orientation of the egg with 

 respect to the future sagittal plane, anterior and posterior end 

 of the larva, etc. ; and the regional metamorphosis or shifting of 

 areas which occurs in consequence of the transformation of the 

 one-layered blastula into the three-layered larva. 



Nothing is known of the orientation of the tg^ until the first 

 maturation spindle is formed; the direction of this indicates 

 the future position of the polar globules and these in turn 

 indicate the position of the anterior end of the trochophore, 

 or the animal pole of the Q.g%\ 180° opposite lies the vegetative 



