THE FIRST SOMITE. 87 



tennal segment) take up a position iu front of and at 

 the sides of the mouth by the end of Stage b. This posi- 

 tion they maintain during the whole of development. Their 

 splanchnic walls are, at first, in close contact with the endoderm 

 of the anterior part of the alimentary canal, and afterwards 

 with the ectoderm of the stomodseum when that is formed 

 (Plate VI, fig. 8). They grow forward into the antennae when 

 the latter aippear in Stage d, so that the bases of the antennae 

 are hollow (Plate VI, fig. 7). Soon — iu old embryos of Stage 

 D (Plate II, fig. 29) — the cells of that portion of their inner 

 wall which adjoins the ingrown ectoderm, proliferate, and form 

 a mass of cells (Plate VII, figs. 16 a, 19 b, ph. m.) which ulti- 

 mately give rise to part of the musculature of the pharyngeal 

 wall and tongue. 



In Stage e, or possibly late in Stage d, the wall of the 

 posterior external corner of the somite becomes markedly thick- 

 ened (Plate VII, fig. 19 b, S. o. \) and pushed out ventrally 

 into a short pouch. In the later embryos of Stage e, and in 

 young embryos of Stage f, this pouch, which is placed imme- 

 diately behind the eye and at the level of the origin of the lip 

 from the pre-oral somite, forms a distinct tube lying along the 

 outer side of the nerve-cord (hind end of brain or beginning 

 of ventral cord), and reaching to and fusing with the ventral 

 ectoderm (Plate VII, fig. 22 b, S. o. 1) immediately in front 

 of the jaw. So far as I can make out, an actual perforation is 

 never formed at the point of contact. The tube persists until 

 the later period of Stage f, being found in embryos in which 

 the cerebral grooves are partly closed (Plate IX, fig. 50, 

 S. 0. 1) . It then vanishes without leaving a trace. 



There can, I think, be but little doubt that this structure is 

 the rudimentary nephridium of the somite. It presents exactly 

 the same relations as do the nephridia of posterior somites ; it 

 is a development of the posterior, external, ventral corner of 

 the somite ; is closely applied to the outer border of the central 

 nervous system, where it fuses with the ventral ectoderm ; and, 

 as we shall see in a moment, the part of the somite into which 

 it opens becomes separated from the remainder. With regard to 



