SCHULTE AND TILNEY, NEURAXJS L\ THE DOMESTIC CAT 331 



a lengthening of its dorsiim which becomes marked and characteristic 

 in later stages. In the hindbrain tlie pontine angle has increased in 

 prominence. The first segment (31) is very large, the second (12) is 

 ventrall_y subdivided into two, as is also the third (13). The sulci 

 corresponding to these subdivisions extend dorsad only so far as the 

 ganglia. Behind the third of the original segments of this region is 

 one small projection confined to the ventral portion of the neural tube. 

 This is followed by the vertical myelomeres corresponding to the meso- 

 dermic somites. 



The large ganglia are shifting their points of connection with the 

 neural tube and are now attached somewhat al)Ove the middle of its 

 lateral wall. The profundus (3a) has shifted to the middle of the 

 second segment, lying at the extremity of the sulcus which marks its 

 ventral subdivisions. The quintal ganglion (3) has lost its primitive 

 intersegmental position and adheres to the cephalic part of the third 

 segment. These two ganglia are thus beginning to approach one an- 

 other. The acoustico-facial (4) occupies the interval between the fourth 

 segment (IJf) and tlie first myelomere (24). 



Embryo of Tiventy-one Somites. — (Plate XXXVIII.) The forebrain 

 differs from that of the preceding embryo notably in the enlargement 

 of the infundibular region (IS) as well as in the increase of the ectoptic 

 zone as a whole. The optic vesicles have receded further from the 

 ventral margin and a broader strip connects the infundibular region 

 with the telencephalon (W). The ectoderm is adherent to the latter 

 at a point corresponding to the somewhat angular junction of the ventral 

 and cranial margins of the pallium. On either side of this line of 

 adherence the neural tube gives rise to projections (.27), not quite sym- 

 metrical, which bear the same relation to the medullary plate as the 

 large ganglia at their inception. The element of the right side, which 

 is somewhat the larger, contains in its interior two small cavities which, 

 however, do not communicate with the lumen of the neural tube. We 

 are unable to offer any suggestion as to the significance of this structure, 

 nor have we found in younger or older embryos of the cat any corre- 

 sponding structure. 



The thalamencephalon (10) and mammillary region (11) together 

 form a well-defined segment of triangular outline interposed between 

 the foregoing structure and the midbrain (15). The latter shows an 

 increase of length in its dorsal zone. The hindbrain differs but little 

 from that of the preceding embryo. Its first segment (-37) is somewhat 

 compressed; the second (12) is subdivided ventrally and the profundus 

 ganglion {3a) is attached at al)0ut its vertical middle, close to its caudal 



