SCHULTE AND TILNEY, NEURAXIS IN THE DOMESTIC CAT 339 



(Plate XXII, Fig. 2; Plate XXIIJ, Fig. 2, and Plate XXVII, Fig. 1). 

 The process is completed iu the embryo of four somites. Two conspicu- 

 ous landmarks are now present — the tubercle of the floor and the optic 

 sulcus. Corresponding to the latter is a thick-walled evagination, the 

 optic vesicle (Plate XXIV). This anlage resembles those of the quintal 

 and acoustico-facial ganglia and is peculiar only in the course of the 

 sulcus, which here approaches the floor at its cephalic and not its caudal 

 extremity, as in the case of the other ganglia. The tubercle of the floor 

 intervenes between the terminal portions of the optic sulci. Yentrad it 



Fig. 2. — Schema of the composition of the encephalon in terms of basal and alar plates 



and ganglionic zone 



1. Basal plate. 2. Alar plate. 3. Sulcus limitans. 4. Ganglionic zone. 



is in relation with the blind extremity of the foregut; the stomodaeum 

 approaches, but hardly reaches it, from in front. By means of these 

 relations the tubercle is easily recognized in succeeding stages, when 

 ultimately it forms a transverse ridge intervening between the mam- 

 millary and infundibular regions. 



The wall of the optic vesicle is divided by the sulcus into a ventral 

 portion adjoining the floor plate and an ectal zone extending to the 

 neurosomatic junction, and therefore forming the summit of the medul- 

 lary fold and later the lateral lip of the neuropore. Ventrally this zone 

 joins the tubercle of the floor, which so constitutes the ventral neuroporic 



