270 FREDERICK TILNEY 
contain extensions of the third ventricle. In amphibia the 
inferior lobes are prominent elements derived from the infundib- 
ular region, while in sauropsids and mammals the tendency to 
the formation of the extensive hypoarium appears to have ceased 
and no corresponding structures are to be found in the position 
of the inferior lobes. Fritsch (17) suggested the homology of 
these structures with the corpora mammillaria, but Herrick (14) 
and others have shown conclusively that this suggestion is not 
well founded. The significance of the inferior lobes becomes 
more apparent in the light of their embryological history. As 
already stated, the infundibular region in embryos of the dog- 
fish, chick and cat is a secondary derivative from the primitive 
optic vesicles. After this region has made its appearance the 
portion immediately caudad to the optic chiasm undergoes 
certain changes which in the selachian terminate in the formation 
of the inferior lobes. The development of these structures as 
definitive elements in the floor of the diencephalon begins at a 
* relatively late period and is characterized by the growth of a 
diverticulum immediately behind the chiasm in such a way that 
two symmetrical evaginations are formed, each containing an 
extension of the third ventricle. These bilateral evaginations 
at first have thin walls but they grow rapidly, the walls becoming 
thicker and the cavity contained within them being correspond- 
ingly reduced in size. In the bird the same tendency to the 
formation of a large diverticulum immediately caudad to the 
chiasm is observed. ‘This diverticulum in the relatively late 
stages tends to become divided into two symmetrical evaginations 
and at this stage resembles in all details the early formation of 
the inferior lobes in the selachian. Thereafter, however, the 
impetus toward the formation of the typical ichthyopsid hypoar- 
ium appears to cease. The evaginations remain comparatively 
thin-walled and finally become a fairly prominent post-chias- 
matic eminence. A similar course of events is observed in the 
mammal as illustrated by the development of the domestic cat. 
Here the infundibular region caudad to the chiasm at first forms 
a diverticulum which later becomes subdivided sagittally in such 
a way as to present two bilateral evaginations. ‘The conditions 
