398 W. H. C4ASKELL. 



large pear-shaped ganglion cells, some of which have relation 

 to the outgoing metameric nerves; (3) masses of closely packed 

 small cells which are presumably nervous in nature ; and (4) 

 a peculiar mass of fine nerve-fibrils, known by the name of 

 the reticulated substance, or " Punctsubstanz/' These two 

 latter elements increase in amount as we approach the supra- 

 oesophageal ganglia. Corresponding to these separate elements 

 we find in the epichordal portion of the brain of the Ammo- 

 ccetes externally — (1) the white substance composed of nerve- 

 fibres; internally the grey substance composed of (2) groups 

 of large ganglion-cells, which are partly the nuclei of the seg- 

 mental cranial nerves, and partly are connected with the large 

 Miillerian fibres ; (3) a thick layer of closely packed small cells 

 in which the groups of larger cells are embedded (see figs. 

 12, 14, PI. XXVI). These small cells are arranged like 

 clusters of berries, and are called therefore by Ahlborn " beeren- 

 formige Zellen ; " their numbers increase enormously, as he 

 points out, when the spinal cord passes into the medulla oblon- 

 gata. (4) Internally, of all in the adult condition, a thick 

 layer of very fine fibrils, interlacing in all directions, which 

 forms the substantia centralis gelatinosa, and which, as already 

 mentioned, increases markedly as we pass from the spinal cord 

 into the medulla oblongata. 



If, as I believe, these four structures are the same as in the 

 nervous system of the Crustacean, then it follows that the 

 supporting tissue of the Ammocoetes' brain is derived from the 

 supporting tissue of the Crustacean nervous system mainly, 

 and only to the slight extent already mentioned, from the 

 modification of the walls of the old stomach. In the sub- 

 stantia centralis gelatinosa we have, on the view put forward, 

 the meeting-place of so many different structures ; here we 

 must find the remains of the connective-tissue elements sur- 

 rounding the stomach, and those which surround the nervous 

 system ; here are the modified extremities of the columnar 

 stomach-cells as already mentioned ; here also is that whorl of 

 fine fibrils partly nervous (corresponding to Gerlach's plexus), 

 partly supporting, known by the name of the reticulated sub- 



