240 



MEAD. 



[Vol. XIII. 



totroch. It is obvious that, when the prototroch has united 

 dorsally, communication is shut off between the general ecto- 

 derm of the two hemispheres. 



I have not observed any further divisions of the rosette cells, 

 now eight in number, and believe that they bear the apical tuft 

 of cilia, which appears long before the interruption of the pro- 

 totroch is obliterated. 



b. Posterior hemisphere. 



Completion of prototroch. — 

 the secondary ti'ochob lasts, a^ 



Shortly after the 64-cell stage, 



■I ^2.1.2 Z,2.I.I /,2.1.2 „2.I.I _2.1.2 „11 



, a , o , o , c , c , all 



divide obliquely to the right in accordance with the alternating 

 rhythm (Figs. 26, 2y, 28). 



These divisions are precisely alike in each of these three 

 quadrants, as shown in text Fig. III. a:^''', b'^'^''', and c^'^'^ divide 



about equally, the others, «^■'■^ b^'^''', 

 c"'^'", very unequally. Of the result- 

 ing four cells in each group, the three 

 larger ones are the secondary proto- 

 trochal cells, which never divide again, 

 but soon become covered with cilia 

 and form part of the prototroch. By 

 virtue of their position they fill the 

 gaps in the primary prototroch, ex- 

 cepting, of course, the mid-dorsal 

 one, which, as we have seen, is filled 

 by the concrescence of the primary 

 prototrochal cells. By the addition 

 of these nine cells, the prototroch is 

 completed, and its twenty-five com- 

 ponent cells may all be recognized 

 even after the larva has begun to 

 elongate (Figs. 57, 59, 61-63, ^^d text Figs. V, VI). The out- 

 lines can always be seen, though difficult to follow in later 

 stages. 



Somatic-plate cells. — In the posterior quadraiit d^'^'^ and d'^''^''^ 

 (= X^ and x^) behave in a manner very different from that of 



Fig. III. — Diagram sho%ving the rela- 

 tion of the cells iu the region of the 

 secondary trochoblasts just before 

 the completion of the prototroch. It 

 applies to the A , B, and C quadrants 

 in both A mphitrite and Clytnenella. 

 The lightly stippled cells /'/=, etc., 

 belong to the primary prototroch; 

 the heavily stippled cells (from the 

 secondary trochoblasts) complete the 

 prototroch. 



