650 
MR. W. H. FLOWER ON THE CEREBRAL COMMISSURES 
there is a vast difference from the brain of the marsupial. The ventricular aperture (0 0) 
is extremely reduced. Its upper border may be properly compared to the fornix, and 
the thickened part of the inner wall (G), above and in front of the small anterior com- 
missure (F), evidently corresponds to the lower part of the septal area and precommissural 
fibres, as well seen in the transverse section. The walls of the hemispheres are in close 
apposition at this part, as the two lateral halves of the septum are in the mammals; 
but a distinct band of fibres passing across the middle line from one hemisphere to the 
other, above the anterior commissure, has never yet been satisfactorily demonstrated. The 
homology of the minute and delicate transverse lamella of nerve-substance, described by 
A. Meckel as situated above the ventricular aperture posterior to the anterior commis- 
sure, is very questionable. 
Great as is the difference between the placental and implacental mammal in the mode 
and extent of the connexion between the two lateral hemispheres of the cerebrum, it is 
not to be compared with that which obtains between the latter and the oviparous verte- 
brate. 
Description op the Plates. 
All, except fig. 3, Plate XXXVI., are from original dissections. For convenience of 
comparison the cerebral hemispheres are reduced to the same absolute length. 
PLATE XXXVI. 
Fig. 1. Inner surface of the right cerebral hemisphere, Human brain. 
Fig. 2. Vertical transverse section (through the anterior commissure), Human brain. 
Fig. 3. Development of the Human brain (after F. Schmidt). I. Sixth week. II. Eighth 
week. III. Tenth week. IV. Sixteenth week. V. Sixth month. 
Fig. 4. Brain of Kangaroo ( Macropus Bennettii) dissected from above, natural size. A 
portion of the extremely delicate great transverse commissure (B) has been 
removed on the left side to show the structures lying beneath it. 
Fig. 5. Brain of Goose. Inner surface of right hemisphere. 
Fig. 6. Brain of Goose. Vertical transverse section. 
PLATE XXXVII. 
Fig. 1. Brain of Sheep. Inner surface of cerebral hemisphere. 
Fig. 2. Brain of Sheep. Vertical transverse section. 
Figs. 3 & 4. Brain of Rabbit. 
Figs. 5 & 6. Brain of Sloth ( Cholcepus didactylus). 
Figs. 7 & 8. Brain of Hedgehog (Erinaceus europceus). 
