NOTES ET REVUE xxv 



become greatly flattened and serve as a protectinj^ oiivclopc for Ihc 

 embryo, until cast ofT at the time of shedding ol" Ihc yolk nicni- 

 brane. In Pliascolosoma they are large cells, heavily ladcn wilh 

 yolk {Hgs3, 4 and 5). They forma girdle around Ihe trochophoir 

 which is incomplète on the dorsal side, where a médian longitu- 

 dinal band of ectoderm cells, the dorsal cord (Fig i rd. d), connecls 

 the apical plate with the ectoderm of the trunk (somalic plate). 



crc.pr'or. 



Fig. 6. 



FiG. 6. — Parasagittal section of Ph. Gouldii immcdiately previous to sbeddiiig of 

 vitelline meiubrane (z. /■.), shovving passage of yolk granules from the degeneraling 

 prototroch into the coelom. 

 crc. pi^or preoral circlet of cilla. 

 mu. rlr. d. dorsal retractor muscle. 

 mu. rlr. v. ventral » » 



The ectoderm of the trunk grows forward and ventrad heneath 

 the prototrochal area, and dorso-lateral proliférations of the apical 

 plate extend backward under the cells of the prototroch, which 

 meanwhile degenerate, their yolk and other metaphismic pro- 

 ducts passing backward into the body-cavity (Fig. 6). The action 

 of the retractor muscles, which hegin to function at this time, 

 facilitâtes the crowding backward of the yolk inio tiie cœlom, 

 which is meanwhile being formed by splitting of Ihe mesobln.stic 



