340 AXXALS \E\V YORK ACADILUY OF SCIENCES 



lip. It is obvious, but important to note, that the optic vesicle at this 

 stage forms the cephalic extremity of the neuraxis. Preoptic structures 

 in consequence must be derived secondarily, cither from the ectal zone 

 of the optic vesicle, or by inrolling of somatic ectoderm at the neuropore. 

 Of the latter process there is no evidence in the cat, and this alternative 

 seems excluded l)y the alirupt character Qf the neurosomatic Junction. 

 We may, therefore, confine our attention to*thc optic vesicles and sum- 

 marize the changes by whicli they give rise to an ectoptic zone wliich 

 includes the anlagcs of the tlialamencephalon, telenco]ilialon and the 

 infundil)ular region. Most striking is the progressive iuul absolute re- 

 duction in size of the optic vesicles, which are actually smaller at tlic 

 stage of sixteen somites tlum they were at eight. Coincidently they cease 

 to occupy the whole vertical extent of the wall of prosencephalon and 

 become relegated to a ventral position. The nature of these changes, 

 especially the reduction in size of the optic vesicle, permits of but one 

 interpretation, namely, that the ectoptic zone is formed at the expense 

 of the vesicle. A similar remodeling of tlie ventro-caudal portion of tlie 

 vesicle gives rise to the infundibular region. 



As the tubercle of the floor constitutes the extremity of the floor-plate 

 and at the same time the primitive ventral lip of the neuropore, it is of 

 prime importance to ascertain its position in subsequent stages of de- 

 velopment. It is easily recognized hy its tliickness and its interposition 

 between the ventral ends of the optic sulci. At first, a]s(\, it is abutted 

 upon by tlie ciil-dr-sac of the foregut which, however, rapidly recedes 

 from it, at eight somites only reaching its caudal extremity and at ten 

 somites terminating beneath the midbrain. 



In Plate XXXIX, Fig. 1, the ental surface of the lu'ain is shown bv 

 a mid-sagittal section of an embryo of eight somites. The tubercle of 

 the floor (2) forms the ventral lip of the neuropore; its cephalic ex- 

 tremity is connected with the supi'asulcal portion of the optic vesicle of 

 each side. From tlie parieties it is separated by the shallow ])rolongation 

 of the optic sulcus which terminates in a depression of the floor, imme- 

 diately behind the tubercle and above the foregut. There is. as yet, no 

 corresponding elevation of the ectal surface; the recess is the first evi- 

 dence, in our series of the mammillary region (17). In tlie embryo of 

 ten somites (Plate XXXIX, Fig. 2) these fundamental relations are 

 still recognizable, although important changes have supervened. Coin- 

 cidently with the ventral deflection of the extremity of the neuraxis, the 

 tubercle of the floor (2) has assumed a vertical position. A considerable 

 degree of closure has been effected ventrally in the anterior neuropore, 

 so that the tubercle no longer constitutes its ventral lip. The mammil- 



