328 AXXALs m:\v yokk academy of sciences 



its fellow, No. 188. Tlie neural folds arc united, except in the short, 

 narrow rhomboid fossa, in the caudal portion of which the neural plate 

 still passes by a gradual transition into the somatic ectoderm. The an- 

 terior neuropore (S) has closed, but the ectoderm is still adherent from 

 the middle of the optic vesicle to the thalamencephalon. Embryo Xo. 

 188 is more advanced in this respect, for the ectoderm is free of the 

 neuraxis in the wliole lengili of the prosencephalon. It is, however, 

 adherent in the midline from the level of the profundus ;nilage to that 

 of the acoustico-facial ganglion. A comparison of these two embryos 

 affords a striking example of the irregularity incident to the whole 

 process of closure of the neural tube and its separation from the ectoderm 

 in the cat, and seems to justify the attachment of less importance than 

 is usually ascribed to the point of ultimate closure. The neuraxis is 

 bent ventrad at the posterior isthmian sulcus (23). This is the second 

 actual flexure observed in our series, for a comparison witli llie figures 

 of the i^receding eml)ryos shows that the earlier projection ventrad of 

 the prosencephalon Avas associated wiili a remodeling of the midbrain 

 and an inclination of th.e anterior istlniiinn sulcus, while in the stage 

 now under discussion the bend is accentuated at the posterior isthmus. 

 The hindbrain forms a gentle arch passing into the straight rayelenceph- 

 alon. In the forebrain important new conditions are initiated. The 

 optic vesicle (1) is now not only relatively but absolutely smaller than 

 in the younger embryos and an ectoptic zone begins to emerge from its 

 periphery. As yet these changes are conspicuous only dorsal ly between 

 the optic vesicle and the thalamencephalon (16), and to a less degree 

 ventrad immediately in front of the mammillary region (77). The 

 dorsal element is the telencephalon (J 9), the ventral corresponds in 

 general to the infundibular region (IS). The mammillary region and 

 the thalamencephalon have increased in size and form well-marked tri- 

 angular prominences in lateral view. The midbrain (15) is a well- 

 marked dilatation, triangular in form, defined by conspicuous isthmian 

 furrows (22, 23) which all but meet ventrally in the angle formed by 

 the second flexure of the neuraxis. The arched form of the hindbrain 

 has been mentioned; its vertical diameter is increased by a ventral pro- 

 jection at the level of the quintal ganglion (5), the pontine angle. Thus 

 it appears that a projection of this region long antccedes the develop- 

 ment of the pontine flexure. In lateral view, the three segments with 

 their oblique separating furrows are more conspicuous than in the younger 

 embryos. They are especially prominent ventrad, which would seem to 

 imply that their increasing definiteness is associated with the bending of 

 the hindbrain. The ganglia are attached dorsally in the furrows; the 



