THE CELL ORIGIN OF THE PROTOTROCH. 121 



?7ZU,C 



secondary prototroch. These finally divide again in the direc- 

 tion indicated by the spindles in Fig. lo. Thus each of the 

 three secondary trochoblasts is at last represented by a group 

 of four cells, q, r, s, and t, in Fig. 12. Three of these, q, r, and 

 s, are about equal in size, and constitute the secondary prototrocJi. 



The relative position of all 

 the prototroch cells is shown in 

 the apical view of the umbrella 

 hemisphere in Fig. 11, which 

 is slightly distorted and brings 

 the secondary prototroch cells 

 into sight. The shading accords 

 with that of the previous figures. 

 The primary prototroch cells are 

 stippled, the secondary shaded 

 with lines, and the minute prod- 

 ucts of the secondary trocho- 

 blasts are drawn with dotted jyinc- 

 lines. The unshaded cells in 

 the upper margin between two 

 groups of primary prototrochal 

 cells interrupt the continuity of 

 the prototroch. They corre- 

 spond in origin, however, to the 

 secondary prototroch cells of the 

 three other quadrants. The re- 

 maining cell, /, Fig. 12 (dotted 



in Fio- II), is small, divides Yio.^s--Amphitrite. Prototroch, paratroch 



^ ' closed veiUrally. ?«2<i:. ^/., mucous glands; 



again much later, and has an ^., Aagella; v.c, ventral band of dlia: 



1 1 ,• --ru^ :^4-^^ /roi., problematic bodies which persist only 



unknown destmy. The inter- ^^ i^^^^.^ , he prototroch. 



ruption in the continuity of the 



ciliated band in the mid-dorsal region is later obliterated by the 



approach and concrescence of the prototroch cells from either 



side. The completed prototroch, consisting of twenty-five 



cells, persists for five or six days, until the larva has developed 



several seta-bearing metameres. 



Some of the later stages of the trochophore are represented 

 in Figs. 13, 14, 15, and 16. For a considerable time the outlines 



