THE DIENCEPHALIC FLOOR PA Give 
period a sharply defined area in the ventral infundibular wall 
clearly differentiated to form not a gland as some maintain but 
a sense organ of unknown significance which is functiona! in 
the early larval stages. Other pelagic teleosts present a similar 
structure, although not as clearly marked as in the Muraenoids. 
The ectoptic zone. It has seemed to the writer that some of the 
discrepancies above mentioned may arise from the fact that the 
‘infundibulum’ in the embryological sense is not one of the primi- 
tive areas of the diencephalon. It is, as already shown in the 
domestic cat by Schulte and Tilney (12), a secondary derivative 
of the primitive optic vesicle; interpreted in this light its signifi- 
cance seems to become more clear. In the domestic cat, the 
chick and the dog-fish two primitive areas may be recognized | 
in the developing forebrain. For a considerable period before 
the neural folds meet in the region of the prosencephalon the 
optic vesicles are the only elements present. They appear at the 
cephalic extremity of the neural folds as prominent diverticula, 
one upon either side. Almost immediately after the formation 
of the vesicles and before the neural folds have met, there appears 
a recess in the floor of the prosencephalon directly caudal of the 
apex of the optic evagination. ‘This recess presents a correspond- 
ing ectal protuberance which forms the mammillary region. 
The optic vesicle and mammillary region are consequently the 
primitive derivatives of the cephalic extremity of the neural 
tube. These observations are true of the cat, chick and dog- 
fish. In the subsequent evolution of the prosencephalon the 
optic vesicles play the more important réle of these two primitive 
derivatives. For a period prior to and for some time after 
closure of the neuropore this vesicle undergoes a profound 
remodelling, as a result of which the optic evaginations become 
reduced in size and an ectoptic zone appears in the form of an 
are about the vesicle. This are presents a dorsal, a cephalic 
and a ventral segment, the latter being the last to make its ap- 
pearance. When, however, it has appeared it constitutes a 
well defined area, the infundibular region. 
It has already been shown (p. 233) that the telencephalon 
arises from the cephalic segment of the ectoptie zone, while the 
