Mr Doncaster, Notes on the Development of Sagitta. 267 



Notes on the Development of Sagitta. By L. Doncaster, B.A., 

 King's College. 



[Read 11 November 1901.] 



O. Hertwig's account of the embryonic development was con- 

 firmed in every way except that in Sagitta bipunctata head-cavities 

 are formed just as Butschli described in 1873. In S. enflata 

 however these cavities are so small as to be seen with difficulty, 

 and Hertwig probably studied a similar species. Sections of the 

 embryo show that in its early stages the nuclei lie at the free ends 

 of the cells, but as development proceeds those of the ventral 

 ectoderm sink into their bases, and in the ventrolateral areas a 

 great proliferation of nuclei takes place, giving rise to the lateral 

 nuclear bands of the ventral ganglion. The cavities of the 

 embryo disappear entirely, and the endoderm becomes reduced to 

 a thin septum, the mesoderm to two solid strands, in which most 

 of the nuclei become aggregated dorsally and ventrally, and the 

 cell-protoplasm beneath them becomes converted into the longi- 

 tudinal muscles. The larva is as described by Hertwig, but he 

 failed to observe the mode of formation of the posterior transverse 

 septum, which arises between the genital cells of each side as 

 they migrate from the splanchnic mesoderm across the body- 

 cavity to the body-wall. This migration takes place at the time 

 of the reappearance of the ccelom, and the septum is probably 

 formed from the mesodermal envelopes of the genital cells. The 

 ectoderm of the larva in the neck region and at the front end of 

 the fins is thickened and consists of vacuolated cells like those 

 composing the epidermis of Spadella draco. No trace of 

 excretory organs nor of genital ducts was found in the larva ; the 

 latter appear only as maturity approaches. There is no ccelomic 

 epithelium, but the muscles are formed from the basal ends of 

 the cells which line the ccelom, as in the Nematoda. This fact, 

 combined with the mode of origin of the transverse septa and the 

 absence of many Annelid characters, supports the view that the 

 Chsetognatha are not related to the Annelida. 



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