376 R. T. GUNTHER. 



The following are the details of the process as they have 

 been described among Mollusca : 



In Chiton, Haller observed the young ova to sink below the 

 ovarial epithelium, and then growing larger bulge it out, so 

 as to form a follicle. By a process of growth each egg-cell 

 gets carried out into the perigonadial coelom on the end of 

 a stalk made by the follicular epithelium. 



In the Cephalopoda the same process obtains. In Nautilus 

 the result is exactly as in Chiton. In Octopus, Argonauta, 

 and others a higher grade of complexity is reached by the 

 formation of branches from the simple egg-stalks, resulting 

 in the production of '' egg-trees." In the Oigopsidte the 

 region that bears the egg-stalks projects right across the 

 genital cavity as a " spindle-shaped body beset all over with 

 stalked eggs." 



In Sagitta the process is as follows : The genital cells in 

 the embryo lie close to the longitudinal partition, bedded in 

 mesoblast, one behind the other (Doncaster, iigs. 14, 17). 

 At a particular stage they become enclosed in a sort ©f 

 cellular envelope of mesoderm, which does not yet form a 

 definite epithelial sheath; a condition to be compared with 

 the stage in Chiton and other Molluscs before stalk-formation 

 has commenced. Fig. 15 of Doncaster's paper shows some 

 of these follicular nuclei closely adpressed to a genital cell. 

 About the fourth day genital cells which have lain quiet for 

 some time " move slowly across the coelomic cavity until they 

 reach the body-wall on each side." While traversing the 

 ccelom the male and female cells on each side move together, 

 and during their progress the transverse septum is formed 

 between them by the cells, which we homologise with 

 the follicular cells of Mollusca. The process of the forma- 

 tion of the transverse septum is therefore absolutely distinct 

 and unlike the process of the growth of septa in Annelids, 

 and although it is impossible to say whether the genital cells 

 themselves or the follicle cells which lie about them are the 

 verse causse of the movement, yet the fact of its occurrence 

 in the two groups we are considering is significant. 



