374 ADAM SEDGWICK. 



The dorsal hump, however, has already begun to atrophy ; it 

 eventually completely vanishes. 



The general ectoderm possesses a large number of vacuoles, 

 and in certain places the nuclei are aggregated together in 

 masses, and are smaller than elsewhere, forming the rudiments 

 of the future spiniferous sense-organs. The latter give rise to 

 the white spots seen on the skin of embryos of this age. 



In P. Balfouri, and, to a slight extent, in P. c a pen sis, 

 the dorsal ectoderm contains a number of highly refractile 

 globules (PI. XXVI, figs. 1 — 4). These are probably yolk 

 globules. They seem to be most numerous in the dorsal hump. 

 The whole dorsal ectoderm of P. Balfouri is thicker than the 

 ventral, and partakes, to a certain extent, of the character of 

 the ectoderm of the dorsal hump. 



In the later stages, in embryos just before birth, the dorsal 

 ectoderm is highly protoplasmic and much striated, and con- 

 tains very few, if any, vacuoles (PI. XXVIII, fig. 12), while the 

 ventral ectoderm is much vacuolated, and retains the cha- 

 racters it possessed at Stage g. That is to say, the nuclei lie 

 in the outer part of the layer, the inner parts being reduced 

 to fine unstained strands passing between the vacuoles. 



The further changes in the general ectoderm need no special 

 description. The nuclei come closer together, the vacuoles 

 disappear, a cuticle is formed on the outer surface, and the 

 adult condition gradually acquired. The claws of the jaws 

 and legs, and the spines of the sense-organs, are special de- 

 velopments of the cuticle. 



The following ectodermal organs require a special description, 

 and will be considered separately and apart from the general 

 ectoderm : 



1. The ventral organs. 



2. The nervous system. 



3. The slime-glands and crural glands. 



On the origin of the trachea I have no observations. 

 They seem to arise very late, and have hitherto escaped my 

 observation,. 



