STUDIES ON THE EMBRYOLOGY OF THE SIPUNCULID,E. 441 



platte), which is to furnish the whole of the definitive ectoderm of the trochophore 

 except the apical plate. 



Simultaneously with this process occurs the formation of the embryonal envelope 

 or serosa. The cells of the body between the apical and somatic plates go to form 

 this membrane. These cells, which are arranged in two or three rows as seen in 

 optical section, are not only shown by their position and number to be prototroch 

 cells, but their probable nutritive function and final dissolution are phenomena that 

 are strangely similar to the function and fate of the prototroch cells of Phascolosoma, 

 as will be shown in the next section. In brief these cells in Sipunculus become flat- 

 tened out against the egg-membrane, spreading backward past the somatic plate 

 till they reach the posterior pole and completely enclose the embryo (Figs. 2, 3, 4). 



In the process of closure the cells become thinner and thinner, in marked dis- 

 proportion to that decrease in thickness which is due to their spreading out. The 

 process of wasting away continues even in later stages, so that Hatschek is inclined 

 to the opinion that the serosa is giving off material which serves to nourish the 

 embryo, a belief which my studies on Phascolosoma tend to corroborate. Mean- 

 while the boundaries of the cells in the serosa and even the nuclei disappear. The 

 cells thus degenerate, and their substance seems to be in part absorbed. 



Even before the closure of the serosa at the vegetative pole the ring-shaped 

 furrow which surrounds the four characteristic cells at the centre of the apical plate 

 (Kopfamnionhohle) is continued backward in the median line by a mid-dorsal furrow 

 (Amnioncanal), which is formed by the sinking of a double row of cells and their con- 

 sequent separation from the zona racliata (Figs. 7, 8). This furrow passes backward 

 into a wide cavity beneath the somatic plate at the posterior or vegetative pole of 

 the embryo (RumpfamnionhOhle) . In other words, all of the ectoderm cells except 

 those which bear cilia have become detached from the zona radiata and sunken 

 beneath the surface; these areas, as I shall show presently, are exactly represented 

 in Phascolosoma by characteristic small cells, which, however, do not sink from the 

 surface. 



The somatic plate of Sipunculus, over which lies the amniotic cavity of the trunk 

 (Figs. 2-6, 8), extends forward in the mid-ventral line of the embryo to the blastopore, 

 and, after the closure of the latter, to the apical plate (Figs. 4, 5). It consists along 

 this ventral side of a narrow tongue-shaped band (Fig. 6, tab. so. v.). At the posterior 

 end of the embryo, and especially on the dorsal side, it is, however, expanded into a 

 broad sheet (tab. so. d.), from which the double row of sunken cells extends forward 

 along the mid-dorsal line through a break in the serosa to the apical plate (Fig. 8). 



It is necessary here to summarize only in part Hatschek's observations, and it 



