THE BRAIN OF A FQITAL ORNITHORHYNCHUS. 183 
region of the cerebrum of pouch specimens of Macropus of 
various ages. These brief notes are all that I know referring 
to the development of the brain of the non-placental mammal. 
In connection with Osborn’s work, one cannot refrain from 
expressing regret that he was so biassed by his previous work 
upon the Amphibian and Sauropsidan cerebrum (Part I of 
his paper) as to misrepresent the condition which obtains in 
the young kangaroo. I am well acquainted with the decep- 
tive appearance which is presented by the brain of the foetal 
Macropus, especially when coloured with ordinary nuclear 
stains—an appearance which lent itself so unfortunately to 
Osborn’s purpose ; but the important position occupied by the 
marsupial in his series should have demanded a more critical 
examination than his paper indicates. For, as the connecting 
link between the whole sub-mammalia—to which his argument 
mainly applied—and the mammalia—to which alone can the 
distinguishing names be applied a priori—M acropus formed 
the link upon which the cohesion of the whole chain of his 
argument depended. By his erroneous interpretation of the 
condition in Macropus his argument lost all its cogency ; for 
in showing, as he clearly did, the homology of that commis- 
sural band, which in the non-placental mammal is undoubtedly 
fornix commissure, with the dorsal commissure of Sauropsida 
and Amphibia, he prepared the way for the statement that the 
corpus callosum—as distinct from the fornix commissure— 
exists only in Placentalia. 
In describing the foetal brain there is, unfortunately, no 
satisfactory account of the adult brain to which one can refer. 
The accounts of Meckel and Zuckerkandl are quite valueless, 
from the fact that their descriptions were based upon very 
badly preserved material. Owen’s description and Garner’s 
notes are reliable as far as they go, but they are very brief and 
general, and deal only with the macroscopic anatomy, as also 
does Turner’s paper. The only attempt at a systematic 
account of the histology—that of Hill—is so clearly biassed 
by its writer’s previous work, that it gives a very erroneous 
and altogether misleading idea of the structures it is supposed 
