142 John H. Gerould, 



of the prototrocli, which form a complete girdle around the egg, 

 except that in the mid-dorsal line the cells in the two dorsal qua- 

 drants come into contact merely at a point, instead of along a line 

 as is the case at the junctions of the other quadrants. 



The cells of the posterior hemisphere at this stage are (1) the 

 daughter cells of the three ventral micromeres of the second quar- 

 tet (2a — 2c), which are to furnish the girdle cells that bear the 

 postoral circlet of cilia in Th. vulgare. (2) the descendants of 2d, the 

 two large cells which give rise to the somatic plate (dorsal in the 

 embryo), (3) the third set of micromeres (3a — 3d), which are to form 

 ectoderm, and (4) the common mother cells of both endoderm and 

 mesoderm (3A — 3D). 



IV. The mesoderm arises from the dorsal representative in the 

 48-64-cell stage of the fourth group of micromeres (viz. 4d). 



V. There is no appreciable segmentation cavity. The trocho- 

 blasts flatten out and crowd backwards over the large cells of the 

 somatic plate, and the endoderm cells become covered by the growth 

 of somatic plate in a sort of epibolic gastrulation. 



Development of the Embryo into the Trochophore. 

 During this period the apical plate, including the definitive rosette 

 with its sensory flagella, is established. The margin of the apical 

 plate sinks slightly beneath the yolk membrane, as in certain 

 Chaetopods, a process which is similar to that by which the amniotic 

 cavity of the head in Sipuncuhis is formed. 



The prototroch is completed by the addition of three secondary 

 prototroch cells, probably the posterior intermediate cells in quadrants 

 A, B, and C. 



Growth of the somatic plate extends chiefly laterad and ventrad 

 by bilateral cleavage, spindles appearing synchronously in corre- 

 sponding cells of the two sides. 



The stomodaeum is formed before elongation begins, by an in- 

 vagination immediately in front of the region of the blastopore, 

 which has been closed by the growth of the somatic plate over it. 



The development of the coelomesoblast is described. Two small 

 cells, probably derived from the teloblasts, are situated close to the 

 posterior, mesial, ventral surface of each teloblast, against the 

 endoderm. Similar cells occur in Chaetopods and in Molluscs, wliere 

 they have been shown to take part in the formation of the wall of 

 the archenteron. 



