100 



John H. Gerould, 



across the egg to the prototroch. The arms of the cross lie in the 

 sagittal and frontal planes of the future embryo. The sister cells 

 of the members of the rosette are situated in the angles of the cross. 

 They are la^-^-~—ld^-^''-, the cross cells of Annelids. 



The spindles which give rise to all of the cells of the anterior 

 hemisphere in the 36— 48-cell stage are laeotropic, except those in 

 the "intermediate" cells, which are radial with a tendency to assume 

 the laeotropic position. Up to this point in the segmentation there 

 has been no deviation from the regular alternation in the directions 

 of the successive cleavages in Phascolosoma, a law of spiral cleavage 

 first pointed out by Kofgii» (1894). 



Not only the rosette, but also the prototroch, are remarkable in 

 Phascolosoma for the large size of their blastomeres. The sixteen 

 cells of the latter are derived by a laeotropic, equal division of the 

 eight trochoblasts of the thirty-two-cell stage. Immediately after 

 the establishment of the blastomeres, they become flattened, and form 

 a broad band, the posterior edge of which slips backward over the 

 adjacent cells, particularly in the dorsal quadrant, where the large 

 cell of the somatic plate, 2d\ sinks beneath the surface, and becomes 

 very nearly covered by the prototroch. The primary cells of the 

 prototroch then form a girdle about the egg, wiiich is complete, 

 except that in the mid-dorsal line the trochoblasts of quadrants Ci 

 and D barely touch each other at one point, instead of being con- 

 nected, as at the three other junctions, along a considerable line. 

 It is the ectoderm of this interruption of the prototroch which I 

 have called in an earlier paper (1903) the dorsal cord, and 

 showed to be homologous to the double row of cells beneath the 

 amniotic canal in Sipunculus. This canal, as described by Hatschek! 

 (1883), passes through a mid-dorsal break in the serosa, or, as I 

 have shown it to be, the prototroch. 



The completed prototroch contains, as is shown by a study 

 of the trochophore, nineteen cells, or three secondary cells in 

 addition to the sixteen primary cells already described. These three 

 cells are probably the posterior intermediate cells of this stage; 

 (-^^i.2.2_^gi.2.2^ Fig. D) which already lie in the gaps between the 

 quadrants in such wise as to complete the band. I have not verified 

 this point by finding cilia on these cells, but three trochoblasts^ 

 which are distinguished from the others by their smaller size, occupy 

 in the trochophore (Fig. 47) precisely the positions of these three 

 cells in the forty-eight-cell stage. 



