THE BRAIN OF A F@TAL ORNITHORUYNCHUS. 187 
bundle, consisting as yet of very few fibres, which also cross 
the middle line in the same lamina. This is the first rudiment 
of the fornix-commissure (fig. 3, f. c.). It will be noticed 
that it lies below the foramen of Monro and in the floor of 
a diverticulum of the third ventricle (figs. 2 and 3), which is 
often known as the ventriculus communis (Osborn). In 
the adult Ornithorhynchus brain the fornix commissure 
occupies a very different position, i.e. in front and above the 
foramen of Monro, and entirely dorsal to the third ventricle. 
This altered position of the commissure in adult Mammalia 
and many Sauropsida probably depends mainly upon the growth 
of the lamina terminalis, especially that part lying between 
the anterior and fornix commissures; but partly also on the 
backward growth of the hemisphere, which is accompanied by 
a corresponding growth of its dorsal commissure. In many 
Amphibia, where the lamina terminalis does not grow to the 
same extent as it does in the higher animals, the fornix 
commissure maintaius into adult life a position exactly cor- 
responding to that met with in the foetal Platypus. In the 
arrangement of their commissures the Sauropsida are inter- 
mediate between the Amphibia and Prototheria.! 
Above the lamina terminalis the median wall suddenly be- 
comes thin again and forms a thin plate, which appears to 
spring from the anterior edge of the lamina terminalis. This 
thin wall (fig, 3, 7. ”.), which is distinguished by von Kupffer 
as the lobus olfactorius impar, bounds a little diverti- 
culum (fig. 8*), which Burckhardt calls the recessus neuro- 
poricus. The same region is distinguished by His as the 
angulus terminalis, and is regarded as the dorsal limit of 
the frontal suture line, and the last point to lose its connec- 
tion with the ectoderm. Immediately dorsal to it there is a 
1 Further and more detailed examinations of the cerebrum of the adult 
Ornithorhynchus and a number of reptiles since this paper was written 
have demonstrated a much closer resemblance between adult and foetus than 
this paper indicates, as well as the marked similarity of both to the higher 
Reptilian condition. A more detailed account of the region of the commis- 
sures will be published in a short time. 
