370 L. DONCASTEU. 



mesoderiu, either in the early or hxter stages of the mlGcratioii. 

 The body-cavity is narrow dorso-ventrally, so that the cells 

 appear to be in contact with both upper and lower walls 

 during- their passage. In many larva? the septum appears 

 from the first single, but it can be seen in some that it is two- 

 layered, for the two layers are not in contact (fig. 21). This, 

 however, would also be the case if it were formed by a fold. 

 As the genital cells cross the cavity they retain connections 

 with the splanchnic mesoderm for a time, apart from the 

 septum; the anterior (female) cell generally has a strand of 

 tissue crossing to the wall of the alimentary canal in front of 

 the septum, and separated from it by a space, while the 

 posterior (male) has a similar strand behind the septum (figs. 

 20, 21). These connections are faintei* than the true septum, 

 and appear to contain no nuclei ; when the genital cells 

 reach the body-wall and take up their permanent position 

 the connections with the splanchnic mesoderm disappear. 



At first there are only very few nuclei in the septum, and 

 one of these generally appears at its outer end, next to the 

 body-wall, and this makes that end of the septum very con- 

 spicuous as soon as it is complete, almost suggesting at times 

 that a fold of the somatic mesoderm is forming, although this, 

 as a matter of fact, does not take place. The nuclei of the 

 envelopes seem to get collected between the genital cells, and 

 so appear in the septum, Avhile on the outer sides of the two 

 cells no nuclei appear during the migration. 



Taking all the facts together, it seems almost certain that 

 the transverse septum is formed in the way here described ; 

 but if it should turn out that it is formed by a mesoblast 

 fold, as suggested by Hertwig, the difference is really of less 

 importance than appears at first. The envelopes of the 

 genital cells are mesoblastic structures, derived from the 

 mesoderm in which the colls have been embedded, so that 

 in either case the septum is formed by a double sheet of 

 splanchnic mesoderm in relation with and in consc([uence of 

 the migration of the genital cells from one side of the body- 

 cavity to the other. In any case the septum is formed 



