THE STOMODJSUM. 67 



protoplasm which contains nuclei of irregular shape. But I 

 still have to describe the development of the stomodseum and 

 proctodseum, and the change produced by it in the adjoining 

 parts of the alimentary canal. 



The enteron at first reaches the front end of the body. 

 Until the end of Stage c it has a considerable pre-oral extension 

 (Plate VIII, fig. 28). The anterior end of the body now 

 becomes retracted, so that in Stage d the mouth lies at the very 

 front end of the middle ventral line (Plate VIII, fig. 29), 

 though laterally the two pre-oral lobes project for a considerable 

 distance (Plate VIII, fig. 32). The result of this is that the 

 intermediate nuclei, which in Stage c extended forwards from 

 the mouth (Plate VIII, fig. 28), now extend backwards and 

 form the dorsal wall of the developing stomodseum. At the 

 same time the lateral walls have appeared as special develop- 

 ments of the same intermediate nuclei (Plate VIII, fig. 32). 



The median ectoderm of the front end of the body so far 

 has been in contact with the front end of the stomodaeum 

 (Plate VIII, figs. 29 and 32). It now separates from it (cf. 

 Plate II, figs. 33, 34) and grows forward, so that a space 

 becomes established between the dorsal wall of the stomodaeum 

 and the front end of the body (Plate VIII, fig. 31). At the 

 same time the dorsal wall of the stomodaeum grows rapidly 

 backwards, while the front end of the enteron maintains its 

 position, or is, perhaps, thrown slightly forward. In this way 

 a blind pocket of the enteron is established, lying on the 

 dorsal side of the stomodaeum (Plate VIII, fig. SI, p.p.). This 

 anterior blind diverticulum persists for some time (late in 

 Stage f) and then disappears without leaving a trace. It has 

 been observed and described by Kennel. 



The stomodaeum, the sides and roof of which are first 

 developed (Plate VIII, fig. 32, and Plate VI, fig. 16 a, b), 

 soon increases in extent, and by Stage f has acquired a well- 

 developed floor (Plate IX, fig. 49). It is now definitely es- 

 tablished, and has a thick lining of oval nuclei and a narrow 

 lumen (Plate VIII, fig. 36). It has also acquired a mesodermal 

 covering from the splanchnic walls of the first and second so- 



