XXIV NOTES ET REVUE 



whicli is about to forin the supra-œsophageal ganglion. A pair of 

 eye-spots (Figs 5, 7 and 8) early make Iheir appearance in the 

 apical plate of the trochophore. 



Tlie nerve cord in the trochophore, immediately before the meta- 

 morpliosis, shows a well-iiiarked, but transitory, division into four 

 metauieres. The sauie is triie of the niesoblastic bands which are 

 simultaneonsly, but transi torily, divided into a corresponding 

 number of somites. 



I hâve not as yet followed the development of the small cells 

 which are early budded ofT from the mesodermal pôle cells in 

 Phascolosoma, as well as in Chaetopods, (Paedomesoblast of Eisig), 

 nor hâve I as yet satistied myself as to the source of the « priniary » 

 mesoderui or inesenchyme, wdiich is présent in the trochophore 

 between the ectotlerm and the mesodermal bands and gives rise to 

 the circular muscles. 



The four permanent retractor muscles are developed in part from 

 certain ectoderm cells situated in small groups upon each side of 

 the apical plate, which become greatly elongated, sink beneath 

 the surface and extend backwartl beneath the prototroch. Four 

 accessory retractor muscles arise in a similar manner from cells 

 which lie between the prototroch and the post-oral circlet. 



Metamorphosis into THE LARVA. The transformation into the 

 larva begins at the âge of about forty-eight hours, when the 

 vitelline membrane (Fig 6, z. r.), which is stretched by the growth 

 of the posterior hémisphère, is torn open, and is thereupon 

 cast ofT, fîrst from the trunk and then from the cephalic 

 (apical) and prototrochal régions. Selenka was doubtless in error 

 when he maintained that, in Ph . elongalum, the yolk membrane 

 persists and becomes the cuticula of the larva. In two species, I 

 hâve watched many times the process of shedding of the mem- 

 brane, and sections of the older trochophores regularly show both 

 the vitelline membrane (zona radiata) and the larval cuticula 

 already formed beneath it (Figs 5 and 6). 



I shall elsewhere show ^ that the prototroch of Phascolosoma is 

 homologous with the serosa or embryonal envelope of Sipunculus, 

 as described by Hatscuek -. In the lalter, the cells of the prototroch 



' Studies on the Embryology of the Sipunculidae. I. The Embryonal Envelope and its 

 Homologue. (Mark Anniversury Volume. Cambridge. Mass. 1904. With plate xxxii). 



* Ueber die Entwicklung von Sipunculus nudus. {Arbeiten a. d. Zool. Inslilut 

 Wien, Bd. V, Heft I, p. 61-140, Taf. iv-ix, 1884). 



