XXII NOTES ET REVUE 



V. There is no appréciable segmentation cavity. The trochoblasts 

 flatten out and crowd backwards over the large cells of the somatic 

 plate, and the endoderia cells become covered by the growth of 

 somatic plate in a sort of epibolic gastrulation. 



TiiE TROCHOPHORE. A typical top-shaped trochophore (Figs. 4 

 and 5) is formed. The apical plate consists of numerous small cells 

 in the midst of which is a group of four larger cells, the définitive 

 rosette (Fig. 4, ros), which arises from the rosette of the 48-celled 

 stage by an arrest in the cleavage of the four apical cells after one 

 or two divisions. This rosette, which lias its homologue in Sipun- 

 culus nudus, bears long sensory llagella. The prototroch is covered 



cdd. 



Vig. 4. 



Fig. 4. — Anterior surface of trochophore of Pfi. vulgare, showing apical plate and 

 complète prototroch, cd. d. dorsal cord of ectoderm. ros, définitive rosette. 



with short cilia, and in Ph. vulgare an isolated postoral circlet of 

 longer cilia is situated behind the prototroch and separated from it 

 by a short interval. This postoral circlet is retained until the larva 

 is aboutfive days old (Fig. 7), the cells of the prototroch having 

 been absorbed in the meantime and the yolk membrane having 

 been shed. A preoral circlet is présent in the larva, and is 

 prominent in the trochophore of Ph. Guuldii which lacks the 

 postoral circlet. 



Growlh of the somatic plate ventrad on each side plays an 

 important part in the development of the trochophore. The cells of 

 the somatic plate divide by bilatéral cleavage. Spindles appear 



