262 BEATRICE WHITESIDE 
tation. Here, too, according to Wiedersheim, the saccus con- 
- tains lime in the embryonic stage. 
If we compare the anatomical structure and the relative size 
of the saccus endolymphaticus in the various classes of Verte- 
brata, we see the following interesting course of development. 
The saccus is present in all Vertebrata, at first as a slight dilata- 
tion at the end of the ductus, then as a large vesicle, and finally 
in the Anura and Ascalabotae as an enormous sac lying in the 
cavum cranii and the vertebral canal (respectively, the shoulder- 
region). From this maximum of development, the size of the 
saccus begins to decrease and at last returns in the Mammalia 
to a hardly perceptible enlargement at the end of the ductus. 
The above-given summary is incomplete because the number 
of animals investigated in regard to the saccus endolymphaticus 
is small. Furthermore, the groups that have been compared do 
not represent a natural phylogenetic line of evolution. There 
are, however, some interesting facts which may perhaps be con- 
nected with some systematic problems. 
Let us first examine the group of the Dipnoi. Here we find 
that Neoceratodus possesses a ductus and saccus endolymphati- 
cus which are very similar to the corresponding structures in 
Polypterus. Thus we can add a new fact to the many points 
of similarity mentioned by Huxley (’76) and Dollo (’95) between 
the Dipnoi and the Crossopterygii. This is particularly inter- 
esting because many investigators, as Huxley, Parker (’92), 
Dean (’92), etc., who took their facts from Retzius, described 
the auditory organ of the Dipnoi as similar to that of the Selachii. 
If, on the other hand, we wish to consider the relationship be- 
tween the Dipnoi and the Amphibia, we find that the recessus 
labyrinthi of Protopterus is very similar in structure to that of 
Rana. 
The large saccus in Protopterus has probably developed from 
the more simply constructed one of Neoceratodus through spe- 
cialization, due, one might assume, to adaptation to terrestrial 
life. Protopterus is generally considered to be a much more 
specialized form than Neoceratodus. Thus Parker (’92) pointed 
out that in Protopterus the reduction of the gills has proceeded 
