SCHULTE AiVD TILNEY, NEURAXIS IX THE DOMESTIC CAT 341 



lary recess (17) is better defined, the mammillary eminence projects 

 laterad but scarcely as yet forms a prominence in the mid-ventral line. 

 As a consequence of closure ventrad in the neuropore and of the ventral 

 deflection of the optic vesicles, a prominent angle is formed immediately 

 below the tubercle of the floor, which now intervenes between the two 

 recesses, the mammillary above and the one just described, the infundibu- 

 lar (18), below. The optic sulcus meets the floor-plate in the infun- 

 dibular region, Ijut a shallow prolongation can still be followed beside 

 the tubercle of the floor. In the embryo of twelve somites (Plate XL,. 

 rig. 1), slight changes have supervened. The floor as a whole is thinner, 

 which may be taken as an expression of the expansion of the cavity, and 

 the tubercle of the floor (2) is no longer a conspicuous thickening. The 

 infundibular region (IS) has increased in size and its cavity is more 

 widely confluent with that of the optic vesicles. Coincidently the termi- 

 nal portions of the optic sulci undergo reduction and lose their intimate 

 relation to the tubercle of the floor. A very faint furrow may be fol- 

 lowed from the optic vesicle across the parieties just above the remnant 

 of the tubercle, while the main line of the optic sulcus extends into the 

 infundibular region. The tubercle of the floor is now losing its de- 

 marcation from the parieties with the effacement of the primitive ventral 

 segment of the optic sulcus, and from this period appears as a transverse 

 ridge inteiTening between the mammillary and infundibular regions. 

 It is, therefore, evident that the mammillary region arises from the 

 cephalic extremity of the primitive floor-plate and that the infundibular 

 region is a derivative of the primitive optic vesicles. 



Kot only ventrally but also dorsally the periphery of the optic vesicle 

 nudergoes a remodeling and important new conditions are established. 

 First, a prominence is formed immediately in front of the anterior 

 isthmian sulcus, the thalamencephalon, and subsequently the telenceph- 

 alon emerges in front of this. The two elevations are separated by a 

 slight depression, the first indication of the velum transversum, from the 

 stage of thirteen somites, the earliest period at which the telencephalon 

 is recognizable. Both of these structures appear in the lip of the anterior 

 neuropore prior to its closure in their respective regions, and are accom- 

 panied by a recession of the optic vesicle from the margin of the medul- 

 lary plate and, what is of major importance, an absolute diminution in 

 the size of the vesicle. This is well marked in the period of from ten 

 to sixteen somites when the thalamencephalon, telencephalon and infundi- 

 bular region are well defined and the optic vesicle reaches the margin of 

 the neuraxis at only a single point between the infundibular region and 

 telencephalon. From this period the ventral pole of the vesicle slowlv 



