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 hipunctata head-cavities 
are formed just as Biitschli 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 w r hich 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 coelom, 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 coelomic 
epithelium, but the muscles are formed from the basal ends of 
the cells which line the coelom, 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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