THE CELL ORIGIN OF THE PROTOTROCH. 125 



found in which the mode of origin of the prototroch is a modi- 

 fication of that in the assumed type, a modification analogous 

 to that in the skeleton of the vertebrates, due either to the 

 fusion or reduplication of parts. 



Fortunately the few annelids in which the cell-origin of the 

 prototroch has been worked out represent different families. 

 Clymenella, for example, belongs to the Maldanidcz, a family 

 not closely related to the Terebelidcz to which Amphitrite 

 belongs. 



The egg of Clymenella is nearly twice the diameter of that 

 of Ampliitrite, contains more yolk, and develops more slowly. 

 The upper hemisphere is relatively smaller, and there are many 

 minor differences in the rhythm of the cleavage and in the size 

 of the cells. The trochophore is similar in many respects, but 

 is less active. The definitive prototroch, however, consists of 

 twenty-five cells, forming a nearly complete band around the 

 larva. In the i6-cell stage we find the four primary trocho- 

 blasts which give rise, by two subsequent cell divisions, to 

 sixteen cells, which constitute the primary prototroch. Each 

 of the three secondary trochoblasts (32-cell stage) divides 

 into a group of four cells, of which three are larger and sub- 

 equal, while a fourth is small. The three larger cells in each 

 group become ciliated and complete the prototroch, while the 

 small one does not enter into it. In brief, tJie origin of tJie 

 definitive prototroch of Clymenella is identical, cell for cell, with 

 tJiat of AmpJiitrite. 



In Arenicola cristata we have a representative of another 

 family of annelids, the Arenicolidce. The eggs of this worm 

 undergo their early development while encased in a gelatinous 

 capsule, and the cleavage differs somewhat from that of Amphi- 

 trite and Clymenella in rhythm and in the relative size of the 

 blastomeres ; but Dr. Child has shown that the primary proto- 

 troch and, later, the definitive prototroch originate in precisely 

 the same manner, cell for cell, as in AmpJiitrite and in Clyme- 

 nella, 



Though, I think, the cell-origin of the definitive prototroch 

 has not been traced in other annelids, there are records of the 

 mode of origin of the primary prototroch in several. 



