THE CELL ORIGIN OF THE PROTOTROCH. 121 



ro 



secondary prototroch. These finally divide again in the direc- 

 tion indicated by the spindles in Fig. 10. Thus each of the 

 three secondary trochoblasts is at last represented by a group 

 of four cells, q, r, s, and /, in Fig. 12. Three of these, g, 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. n, 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 

 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 F 



again much later, and has an 

 unknown destiny. The inter- 

 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. 1 3, 14, 1 5, and 16. For a considerable time the outlines 



par 



II), is Small, divides FlG - I 5- AmphUrite. Prototroch, paratroch 



closed ventrally. muc. gl., mucous glands ; 

 fl., flagella; v.c., ventral band of cilia; 

 prob., problematic bodies which persist only 

 as long as the prototroch. 



