THE AMYGDALA IN AMPHIBIA 269 
amniote brains. But the specificity of the relationship between 
vomeronasal organ and amygdala in the frog suggests further 
lines of interesting inquiry. 
In those reptiles and mammals which possess well-developed 
vomeronasal organs is there a similar specific relationship with 
any component of the amygdaloid complex? 
In the course of amphibian evolution the first component of 
the amygdaloid complex to emerge in morphologically recog- 
nizable form is the anuran amygdala as described in the preceding 
pages, and this apparently occurred under the specific influence 
of the vomeronasal system of peripheral connections. In rep- 
tiles and mammals, as indicated by Johnston, Crosby, Elliot 
Smith, and others, other formations from different sources are 
added to the amygdaloid complex in various patterns. Clearly, 
then, the structure here designated in the brain of the frog as 
amygdala cannot be homologized with the whole of the amyg- 
daloid complex of mammals. This has various other elements 
which may be represented in the urodele ventrolateral and dorso- 
lateral areas in undifferentiated form. 
Johnston (715, p. 419), in connection with his description of 
the amygdaloid complex of reptiles, has shown that the mamma- 
lian amygdala is a composite of elements of diverse origin. The 
definition of the mammalian amygdala is, accordingly, very diffi- 
cult, and in the present state of our knowledge it is of doubtful pro- 
fit to attempt to fix the mammalian homologies of the anuran 
amygdala. In reptiles, however, this can be done, as we have 
seen (p. 236) with some measure of probability (cf. Johnston, 
15, p. 418, and Crosby, ’17, p. 349), though the absence of the 
vomeronasal organ in Crocodilia (Zuckerkandl, ‘10, p. 35) leaves 
this question in some obscurity. 
The amygdaloid complex of mammals has been the subject of 
numerous investigations, of which the most extensive is that of 
Volsch (06, 710). This author, like Cajal (11, p. 724), denies 
direct connection of the olfactory tract with any part of the 
complex in mammals. This, I think, is very questionable, for 
my own observations, mentioned below, suggest that the lateral 
olfactory tract does reach one part of it (the presubicular area), 
