380 ADAM SEDGWICK. 



PI. XXVI, fig. 5, nenro-musc. The nerves, therefore, are 

 to be regarded as special differentiations of a pre-existing 

 network, the origin of which is not known, but which at first 

 pervades and is continuous throughout the whole ectodermal 

 and mesodermal tissues of the body. 



Slime-Glands and Crural Glands. 



The slime-glands are entirely ectodermal products. Their 

 early development has been described in Part III, p. 482, and 

 the later changes being simply processes of growth, I have 

 nothing to add to the account there given. 



The crural-glands appear very late. In embryos of April 

 almost ready to be hatched they have the form of shallow 

 invaginations of ectoderm immediately external to the opening 

 of the nephridium (PI. XXVII, fig. 11). They seem to be 

 entirely derived from the ectoderm, but I have no details as to 

 their development. I could find no trace of the enlarged 

 crural gland of the last leg of the male in the oldest embryos 

 which I have examined. 



The Stomodaeum and Proctodeum— The early history 

 of the sestructures has already been given in Part III, pp. 

 482, 485, 486. 



The mesodermal investment of the anterior part of the 

 stomodseum becomes very thick, while that of the posterior 

 part remains comparatively thin. The lining cells secrete a 

 cuticular layer. The anterior part becomes the pharynx, and 

 the posterior the oesophagus of the adult. 



The proctodeum also acquires a cuticular lining aud a well- 

 marked mesodermal investment. It becomes the rectum of 

 the adult. 



The Endoderm. 



In Stage g the endoderm is reduced to a layer of extreme 

 tenuity (PL XXVI, fig. 5). It soon, however, begins to in- 

 crease in thickness, and in embryos almost ready for birth has 

 the form represented in PI. XXVII, fig 11. The nuclei are 

 placed in the deeper parts of the layer, and the protoplasm 



