THE DIENCEPHALIC FLOOR 259 
in contact with the ectal surface of this evagination, except for 
a small area immediately ventrad to the dorsal evagination. 
In these respects the development of Mustelus corresponds in 
all details to that of the cat and of the chick. The post-infundib- 
ular evagination (36) is the smaller of these secondary protuber- 
ances. Upon its ventricular surface it is demarcated from the 
mammillary region by a distinct elevation, the tubercle of the 
floor (46). Ventrally it becomes continuous with the ventral 
infundibular evagination (78). 
The mammillary region (25) has attained a sharper outline 
without appreciable increase in size. It is assuming the char- 
acteristics which lead to the recognition of it as the posterior lobe. 
The optic vesicle is relatively smaller than the forebrain; its 
peduncle (30) has become more constricted except at its proximal 
extremity where it presents a slight dilatation into which ex- 
tends a recess accessory to the ventricle. The optic peduncle 
still retains its canal which communicates with the residual 
lumen of the optic cup. The optico-infundibular groove is less 
well defined than in the earlier stages. 
Mustelus embryo of 20 mm.; Specimen No. 730 (figs. 26). In 
this embryo the anterior and posterior isthmian flexures are 
present and well marked. All of the elements described in the 
11 mm. embryo may be recognized and are but little changed. 
The principal advances are seen in the thalamencephalon and 
telencephalon. In the infundibular region, however, a process 
of importance has been initiated and has already assumed 
considerable proportions. During the earlier stages it has been 
in the apex or caudal portion of this region that notable changes 
were observed. Now and for some time to follow the develop- 
ment of its cephalic portion becomes more conspicuous and 
salient. Following the ventral border of the optic peduncle, 
some optic fibers have already made their way inward to form 
the chiasm (4); the floor of the ventricle at the same time has 
been slightly elevated above the chiasm as a transverse ridge 
passing between the orifices of the optic peduncles. This ridge, 
the chiasmatic process (2), separates the prechiasmatic and post- 
chiasmatic recesses. Of these the latter is the more prominent 
