126 CHANGES FROM STAGE G TO BIRTH. 



the dorsal ectoderm contains a number of highly refractile 

 globules (PI. X, figs. 1 — 4). These are probably yolk glo- 

 bules. 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. XII, fig. 13), 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 TRACiiEiE I have no observations. 

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

 observation. 



The Ventkal Organs. 



For the origin and general history of these organ I must 

 refer back to Ch. Ill, p. 59, and to Kennel (No. 30). They 

 consist of segmented thickenings of the ectoderm, placed be- 

 tween the appendages and composed of two halves, which 

 are in contact in the middle ventral line (PI. X, fig. 5). 

 During Stage g they possess two kinds of nuclei : the surface 



