136 BIOLOGICAL LECTURES. 



as may be seen from Diagram C. To show the position of this 

 cell in Amphitrite, I have marked it x in Fig. 10 in one quad- 

 rant, and in each of the four quadrants in Fig. 11. If my 

 observations on Amphitrite and Clymenella and those of Child 

 on Arenicola are correct, this cell does not enter the prototroch 

 in these annelids. However, I have never followed its career 

 in either form, except in the posterior quadrant, and here 

 Heath has called my attention to the fact that one of the 

 derivatives of this cell in AmpJiitrite is excessively minute (not 

 as large as a polar globule), and has the appearance, therefore, 

 of a "vestigial cell." As the matter stands at present, this is, 

 I believe, the first important well-sustained discrepancy in the 

 origin of the prototroch, in all the Ti'ocJiozoa which have been 

 studied, and for this reason I would recommend the investiga- 

 tion of the destiny of this cell in the annelids as a fruitful 

 problem in cell-lineage. 



Diagrams B and C in Fig. 25 show that all four derivatives 

 of the secondary trochoblast q, r, s, and t enter the prototroch. 

 This is true in three quadrants. It will be recalled that in the 

 three annelids in which these cells are known, / is not a proto- 

 trochal cell, but is minute and of a vestigial appearance. It is 

 shown in Figs. 11, 12, and 24. 



The cell y in Fig. 25, Diagram B, remains to be discussed. 

 It divides once, and the two daughter-cells take their position 

 in the prototroch, as seen in C. This cell y, like the cell x, 

 does not enter the prototroch in AmpJiitrite (see Fig. 11), 

 Clymenel/a, and Arenicola. It is small in AmpJiitrite , but not 

 minute enough to suggest that it is a vestigial cell, though one 

 of its products is distinctly of this appearance. In respect to 

 the cells y, y', t, and s, however, there is an important fact to 

 mention. They are, as far as could be observed, not ciliated, 

 and probably serve, Heath suggests, as a sort of supporting 

 layer. For this reason it may, perhaps, be a question 

 whether they should be considered part of the prototroch. 

 If we throw them out, however, there arises another dis- 

 crepancy between this prototroch and that of the annelid, 

 for in the latter the cell i- is undoubtedly a part of the 

 prototroch and is ciliated. 



