STUDIES ON THE EMBRYOLOGY j OF THE SIPUNCULID.E. 443 



make their appearance upon the sixteen large "primary" prototroch cells,* and a 

 little later a tuft of long flagella appears upon the apical plate. The latter consists 

 at this stage (Fig. 9) of a comparatively large rosette in the angles of which are four 

 cross-cells, while radiating outward from its four points are two intermediate cells in 

 each quadrant. Thus the active half of the egg in the 48-cell stage consists not of 

 24 cells but of 32, leaving only 16 cells at the vegetative pole. 



The changes which ensue at the anterior pole in the establishment of a complete 

 trochophore involve the division of the cross and intermediate cells into a large num- 

 ber of very small cells. The rosette cells, however, divide probably only once, leaving 

 a definitive diamond-shaped rosette composed of four comparatively large cells which 

 give rise to long sensory flagella (Fig. 11, ros.). This definitive rosette becomes sur- 

 rounded during the next ten hours by the small cells of the apical plate, like an island 

 in the midst of a circular pool (Fig. 11). A dorsal cord (Fig. 12, cd. d.) composed of 

 similar small cells extends backward in P. vulgare from the apical plate through 

 a mid-dorsal break in the prototroch to the somatic plate behind the prototroch. 

 Thus the ectodermal areas in the embryo of Phascolosoma correspond in all respects 

 to those in Sipunculus (Figs. 7, 8), except that in Phascolosoma the cells which sur- 

 round the definitive rosette, those of the mid-dorsal cord, and those of the somatic 

 plate of ectoderm do not sink away from the zona radiata to form amniotic cavities 

 as the corresponding cells do in Sipunculus. f That the latter should sink away from 

 the surface in Sipunculus is not extraordinary, since even in the early cleavage stages 

 the blastomeres are separated from the egg-membrane (zona radiata) by an obvious 

 space. The blastomeres of the vegetative pole in Sipunculus apparently never touch 

 the adjacent zona radiata. The yolk-laden egg of Phascolosoma on the other hand 

 completely fills the zona radiata; and all of the blastomeres, whether ciliated or not, 

 are at all times closely applied to the inner surface of the yolk-membrane (compare 

 Figs. 1 and 10). 



Having observed that the cells of the apical plate, those of the mid-dorsal cord, 

 and those of the somatic plate correspond closely in the two forms, the only other 

 ectoderm cells that remain to be considered are those of the prototroch of Phascolo- 

 soma and of the serosa of Sipunculus. It is impossible to compare these two struc- 

 tures cell by cell until we have some knowledge of the cell lineage of Sipunculus, 

 but there cannot be the slightest doubt that the serosa in Sipunculus represents in 



* A fuller account of the lineage of the prototroch cells in Phascolosoma, including the three "secondary" 

 cells, will be presented in a later paper. 



t It is hardly necessary to call the reader's attention to the striking resemblance between the sipunculid and 

 the annelid trochophore, as regards the arrangement of the ectoderm. 



