504 ADAM SEDGWICK. 



5. The somites of the anal papillae, or twenty-first pair. 



6. The rudimentary somites behind the twenty-first. 



The heading 4 will have to be further subdivided according 

 as the dorsal divisions contain generative cells or not. 



The Somites of the First Pair (somites of pre-oral or au- 

 tennal segment) take up a position in 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 storaodseum when that is formed 

 (PL XXXIV, fig. 8). They grow forward into the antennae 

 when the latter appear in Stage d, so that the bases of the 

 antennae are hollow (PI. XXXIV, fig. 7). Soon — in old 

 embryos of Stage d (Part I, fig. 29) — the cells of that portion 

 of their inner wall which adjoins the ingrown ectoderm, pro- 

 liferate, and form a mass of cells (PI. XXXV, figs. 16 a, 

 19 b, ph, m.) which ultimately give rise to part of the muscu- 

 lature 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 (PI. XXXV, fig. 19 b, S.o.l) 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 (PI. XXXV, 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 (PI. XXXVII, fig. 5 o, 

 S.o.l). 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 



