450 STUDIES ON THE EMBRYOLOGY OF THE SIPUNCULIDjE. 



(C) the somatic plate, which, owing to the yolk within the endoderm, in Phas- 

 colosoma never sinks away from the zona radiata as in Sipunculus; 



(D) the prototroch, which in Sipunculus spreads out forward over the edge of 

 the apical plate and backward over the somatic plate and forms the serosa. In Phas- 

 colosoma the sixteen huge primary prototroch cells are derived from the anterior 

 half of the egg (first group of "micromeres," which, however, in the 8-cell stage exceed 

 in size the "macromeres" except in quadrant D). They become flattened out against 

 the zona radiata as in Sipunculus, though to a less extent. They are covered with 

 the short adoral cilia, and contain the greater part of the yolk of the entire trochophore. 



2. The prototroch cells in Phascolosoma undergo rapid dissolution from within 

 outward, at the time when elongation of the trunk begins and shedding of the zona 

 radiata occurs. Their substance, which has been largely converted into yolk, is now 

 passed into the fluid of the newly formed ccelom, whence it is gradually absorbed 

 during the growth of the larva. The serosa of Sipunculus according to Hatschek 

 also appears to be in some measure a nutrient organ, though its remnant is cast off 

 with the zona radiata, and not passed into the ccelom. 



In Phascolosoma, as in Sipunculus, the dorsal ectoderm of the somatic plate 

 grows forward and ventrad on each side of the body, closing over the region vacated 

 by the prototroch (=the serosa) and joining two dorso-lateral proliferations of the 

 apical plate which take part with it in the closure. 



3. The prototroch of Phascolosoma and the serosa of Sipunculus have probably 

 arisen from what may be called a typical prototroch, such as occurs in mesotrochal 

 annelids, from which type the prototroch in Phascolosoma departs less than does 

 that in Sipunculus. 



The differences in the structure and fate of the prototroch in the two forms appear 

 to be the immediate result of the presence or absence of yolk. Reasons are presented 

 for believing that the ancestors of Sipunculus were provided with a yolk-laden pro- 

 totroch, like that which occurs to-day in Phascolosoma. 



VIII. BIBLIOGRAPHY. 



Eisig, H. 



'98. Zur Entwicklungsgeschichte der Capitelliden. Mitth. Zool. Sta. Neapel, Bd. 13, Heft 1-2, pp. 1-292, 

 Taf. 1-9. 

 Gerould, J. H. 



:04. The Development of Phascolosoma (Preliminary note). Arch, de Zool. expeVim. et g6n., notes et 

 revue, s6r. 4, torn. 2, no. 2, pp. xvii-xxix. 

 Hatschek, B.< 



'78. Studien ftber" Entwickelungsgeschichte der jjAnneliden. Aib. zool. Inst. Wien, Tom. 1, Heft 3, pp. 

 277-404, 8 Taf. 



