278 FREDERICK TILNEY 
the recessus lobi posterioris. In birds and reptiles the develop- 
mental history of the mammillary region through the early 
stages is similar to that in the fish. Later, however, the walls 
of this region begin to thicken rapidly and the mammillary 
recess becomes progressively reduced in size until it is obliterated 
and the solid mammillary bodies have been formed. The 
marked cephalic flexure in the fowl causes a divergence in the 
long axes of the mammillary bodies caudo-cephalad, so that 
these structures do not present the same prominence here as 
they do in the diencephalic floor of mammals. The develop- 
ment of the mammillary region in:the cat manifests certain 
peculiarities which I was unable to observe in either the chick 
or the dog-fish. These peculiarities appear in the formation 
of two relatively early diverticula, the median mammillary 
evagination and the dorsal mammillary evagination. The 
latter evagination is unquestiongbly involved in the formation 
of the corpus interpedunculare, for I have found that the fascic- 
ulus retroflexus of Meynert may be seen passing from the 
_habenular region directly to the evagination in question as early 
as the stage of 25 mm. There can be no doubt that this dorsal 
evagination, therefore, is the anlage of the corpus interpeduncu- 
lare. That no similar evagination has been found either in the 
dog-fish or in the chick may argue that a less definite portion 
of the mammillary region gives rise to the corpus interpedunculare 
in these forms, but it is probable that this ganglionic body takes 
origin from the primitive mammillary region, even though no 
distinct evagination of its own is formed. To establish this 
supposition, however, it will be necessary to study the develop- 
ment of this region further, particularly with a view to the 
ontogenesis of the fiber tracts connecting the several centers 
involved. In the light of these facts, with the exception of this 
interpeduncular element, it seems warranted to. homologize 
the ichthyopsid posterior lobe with the mammillary bodies of 
sauropsids and mammals. 
