THE CELL ORIGIN OF THE PROTOTROCH. i^X 



s 



trochoblasts arise in exactly tJie same manner as in the annelids 

 named, and most, if not all, of their descendants, enter into the 

 prototroch. This general statement should be qualified by an 

 account of at least some of the details of the later cleavage, 

 in order that it may give a fair idea of the degree of resem- 

 blance in question. For this purpose, and to avoid technical 

 expressions, we will make free use of diagrams and arbitrary 

 symbols. 



The secondary trochoblast in each of the three quadrants 

 lies between the primary prototroch cells, as in Diagram 24, 

 which is from Amphitrite (compare Figs. 8, 10, and 12). It is 

 represented by the compound symbol grst. The phenomena 

 are the same in three quadrants, but the corresponding cells 

 in the fourth quadrant never enter the prototroch. In both 

 Amphitrite and Crepidida each trochoblast q, r, s, and / divides 

 obliquely in the same manner into two cells, qr and st, the 

 former lying a little anterior to the latter (Fig. 24 B). Later, 

 in both animals, each of these two cells divides in the reverse 

 oblique direction, making a group of four cells, q, r, s, t (Fig. 

 24 C). In the annelid q, r, and s, soon after their formation, 

 become ciliated and form a part of the prototroch in the manner 

 already described, while /, a very minute cell, does not do so. 

 In Crepidula q and r enter the prototroch ; s divides also, its 

 upper product probably entering the prototroch, while the 

 lower product may enter it or may not. The remaining cell, 

 /, is very much larger than in Amphitrite, and at least part of 

 it probably enters the prototroch, thus giving rise to the only 

 probable discrepancy in the destiny of all the descendants of 

 the primary and secondary trochoblasts in the two forms. 

 What the significance of this discrepancy may be, I will not 

 venture to say, but I wish to call attention again to the fact that 

 the cell / in Amphitrite is exceedingly small, does not divide 

 for a long time, and has the appearance of a vestigial cell. 



It must be said here that Conklin thinks it possible, although 

 by no means certain, that cells other than the trochoblasts enter 

 into the formation of the velum, and in respect to many of 

 the cells which we have already described there is also an 

 element of uncertainty, since the velum of Crepidula does not 



