1 32 CHANGfES FKOM STAGE G TO KlUTH. 



Slime-Olands and Crural Glands. 



The slime-glands are entirely ectodermal products. Their 

 early development has been described on p. 64, and the later 

 changes being simply processes of growth, I have nothing to 

 add to the account there given. 



The Crutal-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. XI, 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 Proctodaeum— The early history 

 of tliese structures has already been given on pp. 64, 67, 68. 



The mesodermal investment of the anterior part of the 

 stomodaeum 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 proctodaeum also acquires a cuticular lining and 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 (PI. X, fig. 5 J. It soon, however, begins to increase 

 in thickness, and in embryos almost ready for birth has 

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

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

 stains deeply and contains a large number of granules. The 

 endodermal part of the alimentary canal is without glandular 

 appendages of any kind. In old embryos the enteron generally 

 contains a deeply-staining material with a number of highly 

 refractile particles in suspension. This substance is probably 



