ON THE DEVELOPMENT OF fSAGlTTA. 369 



this septum is formed is not absolutely clear, for the small size 

 of the animal and the extreme minuteness of the cells made the 

 actual process difficult to follow with certainty. There are 

 two possible means by which such a septum might arise : 

 first, by the splanchnic mesoderm rising up as a fold, and 

 carrying the genital cells across with it till they reached the 

 body-wall (or, what would amount to the same thing, the 

 genital cells moving across and drawing the splanchnic 

 mesoderm with them in the form of a fold); secondly, the 

 septum might arise by the coalescence of cellular envelopes 

 in which the genital cells are enclosed. Such envelopes were 

 described and figured by Hertwig, but in the species which I 

 have studied they have been exceedingly difficult to make 

 out, and I have only rarely been able to see them. By 

 comparing a number of larvas, both mounted whole and in 

 sections, there seems to be no doubt that the genital cells 

 are enclosed in a membrane which is separate from them, 

 and which contains here and there a few nuclei (PL 21, fig. 

 32). The nuclei are much less numerous than those repre- 

 sented in Hertwig's figures, but they indicate that the 

 envelope is a cellular structure, which is no doubt derived 

 from the mesoderm in which the genital cells have been 

 embedded since an early embryonic stage. When the 

 genital cells move across the body-cavity their envelopes are 

 elongated transversely to the body of the animal, and 

 between the two cells their respective envelopes lie parallel 

 with one another, almost, if not quite, in contact. If when 

 the migration of the genital cells begins their envelopes 

 remain attached to the splanchnic mesoderm at the point 

 between the genital cells, while elsewhere they become free 

 and move across with the cells which they enclose, wheu the 

 latter have crossed the cavity and reached the body-wall a 

 two-layered septum Avill have been produced across the body, 

 with the two genital cells lying on opposite sides of it. 

 After comparing a large number of larvie, alive and stained, 

 it appears to me that the septum is formed in this way, for I 

 have never seen any indication of a fold of the splanchnic 



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