THE CELL ORIGIN OF THE PROTOTROCH. 119 



age, one-half of all the cells are descendants of the four upper 

 cells of the 8-cell stage (the anterior hemisphere or umbrella), 

 and that the sixteen primary prototroch cells all belong to this 

 hemisphere. 



By means of the primary prototroch, which consists of four 

 isolated groups of cells, the Qg^, or trochophore, as we may now 

 call it, swims about for a considerable time. In the course of 

 a few hours, however, nine more cells, seco7idary prototroch, 

 become differentiated and develop cilia, and these, together 

 with the sixteen cells already described, constitute the defini- 

 tive p7^ototroch. As is shown in Fig. 

 9, these additional cells, which are 

 shaded with lines, are disposed in 

 such a manner that they fill the 

 spaces between the isolated portions 

 of the primary prototroch except in 

 one quadrant. Thus the definitive 

 prototroch consists of twenty-five cili- 

 ated cells which lie in a nearly com- 

 plete belt aronnd the trochophore 

 (Fig. 13, p. 120). 



The primary and the secondary 

 prototroch cells, although together 

 they form a single larval organ, have 

 different modes of origin, for the 

 cells of the secondary prototroch are 

 derived from the lower (posterior) 

 quartette in the 8-cell stage, that is, from the posterior hemi- 

 sphere, or subumbrella. 



For convenience of description, we may assume the existence 

 of a specific material which will become the secondary proto- 

 troch, and may locate this material in the subumbrella during 

 the successive stages of cleavage. In the i6-cell stage the 

 material occupies a portion of three of the cells of the third 

 quartette, counting from the upper pole, which alternate with 

 the primary trochoblasts. In Fig. 7, p. 117, the cell marked a^ 

 and partially shaded with lines is one of the three cells in 

 question. The cell marked d'^ belongs to the quadrant which 



Fig 



— Diagram of prototrochal 

 cells, seen from left side, in A mphi- 

 irite, Clynienella, and Arenicola. 

 Two groups of primary and one of 

 secondary prototrochal cells. 



