SCHULTE AXD TILXEY, XEURAXI8 IN THE DOMESTIC CAT 325 



pore (5), there are three small gaps (.9) in the region of the quintal 

 anlage (3) corresponding to the hiatus there present in the embryo of 

 eight somites. Another small orifice (10) is situated in the region of the 

 acoustico-facial anlage (4). Caudal to this the folds unite for a consid- 

 erable distance but again separate (11) in the region of the fourth to the 

 eighth somite ; opposite the ninth they are again united for a short dis- 

 tance when they finally separate and diverge. 



The anterior neuropore is markedly diminished in extent. That this 

 is caused by closure at its ventral as well as at its dorsal lip is shown by 

 a comparison of the models in Plate XXVII, Fig. 3, and Plate XXXI, 

 Fig. 2. I]i the eight-somite embryo the optic vesicle is open in its whole 

 extent ; in that of nine somites closure has been effected in about half of 

 its length. The prosencephalon shows a distinct advance beyond that of 

 the eight-somite embryos both in size and in the complexity of its surface 

 relief. It projects strongly ventrad, its caudal margin forming approxi- 

 mately a right angle with the floor of the neural tube. It is demarcated 

 from the midbrain by a shallow anterior isthmian sulcus {22} which is 

 very obliquely inclined. The optic vesicles form its ventro-cranial region ; 

 they are somewhat pyriform with a pointed caudal extremity and their 

 long axes are inclined at an acute angle with the horizontal. Dorsally 

 the optic vesicle is defined by a shallow depression; between this and the 

 anterior isthmian sulcus are two small elevations, one on the dorsal and 

 one on the ventral aspect of the tube. The dorsal eminence is the thala- 

 niencephalon (Plate XXXI, Fig. 2, 16) and is opened in its whole length 

 l)y the neuropore. It is somewhat triangular in shape and its prominence 

 diminishes ventrad where it is separated by a faint depression from the 

 ventral eminence. This latter also has a triangular shape and is the first 

 indication, in our series, of the mammillary region (Plate XXXI, Fig. 2, 

 17). Ventrally it is separated from the pointed extremity of the optic 

 vesicles by a slight incisure which corresponds to a thickening and an in- 

 ward projection of the floor-plate. As this is interposed between the 

 ventral extremities of the optic sulci, it is evidently the tubercle of the 

 floor (Plate XXXIX, Plate XL, Fig. ^,2) oi the earlier stages. The de- 

 pressions defining the thalamencephalon and the mammillary region form 

 an H-shaped system of furrows, while the two elevations taken toge,ther 

 form a segment which separates the primitive optic vesicles from the mid- 

 brain. That this segment is not a neuromere in any precise sense of the 

 term is obvious from its developmental history, for the two eminences of 

 which it is composed fuse only at a considerably later period (sixteen 

 somites). The prosencephalon lies immediately in front of the foregut 

 and the entoderm is closely applied to the mammillary region. A com- 



