THE BRAIN OF A’ F@TAL ORNITHORHYNCHUS. 195 
advanced to explain it. As yet, however, its significance is 
veiled in mystery. In this brain it is enclosed in a sheath of 
dura mater formed by the splitting of the falx cerebri (fig. 7, 
fe.), and it is closely surrounded by a number of very large 
vessels. From the close relation of the organ to these large 
vessels, it has been suggested that it may have some nutritive 
function before the “ diaplexus” is developed (Gage). 
The Relation of the Roof (Deckplatte) to the 
Third Ventricle and Foramen of Monro. 
In describing the foramen of Monro, it has already been 
seen that the Deckplatte (“‘Scheitelplatte” of Burckhardt) is 
separated from its dorsal aspect by the whole width of the 
thalamic rudiment. The foramen opens into the small dorsal 
recess (fig. 15, d.) of the third ventricle which is bounded an- 
teriorly by the angulus terminalis and the lamina supra-neuro- 
porica. The latter structure is considered to form part of the 
Deckplatte, and the continuation backwards of its horizontal 
part in this specimen (fig. 3) gives rise to the choroid plexus 
of the lateral ventricle (fig. 15). In fact, it is generally con- 
sidered that the lateral plexus is a derivative of the Deckplatte 
(Minot). IRf this be so, it must be admitted that the foramen 
of Monro and the hemisphere are in immediate relation to the 
Deckplatte. Ifthis be so, the different parts of the morpho- 
logical roof will be seen to be widely separated in the region 
of the foramen of Monro (fig. 10). Above there is the un- 
doubted roof of the ‘‘aula;” whereas laterally, between the 
thalamic rudiment, and the hippocampal rudiment, is the 
lateral plexus whose epithelial covering is supposed to bea 
derivative of the Deckplatte (Minot). If, however, the hemi- 
sphere, as is generally supposed, extends up to the Deckplatte, 
how does the optic thalamus come to occupy the position 
which is roughly represented in fig. 10, ¢h.? Kven admitting 
that the choroid plexus of the hemisphere is a derivative of the 
roof which has extended backwards, invading, as it were, the 
mesial hemisphere wall, ouly two alternatives appear to remain 
