THE BRAIN OF A FQITAL ORNITHORHYNCHUS. 193 
large mass of neuroblasts (fig. LO, ¢h.), the rudimentary optic 
thalamus. As this mass is traced caudally (figs. 11, 12, and 
13) it gradually increases in size, forming a large bulging of 
the Fltiigelplatte in the posterior part of the diencephalon. 
Where it forms the dorsal boundary of the foramen of Monro 
(fig. 11) this thalamic rudiment consists of three distinct 
parts—a mesial ependymal layer, an intermediate neuroblastic 
mass, and a lateral nerve-fibre layer. ‘The connections of the 
latter cannot be accurately traced, but it appears to be the 
tenia thalami. 
As this thalamic rudiment is traced forwards above the 
foramen of Monro the neuroblastic mass gradually diminishes 
in size, until at the extreme anterior limit of the foramen of 
Monro (fig. 15) it disappears altogether, only the ependymal 
and nerve-fibre layers being left. The nerve-fibre layer also 
soon disappears, leaving the ependymal layer only. 
The point at which the thalamic rudiment ceases corre- 
sponds exactly to the anterior limit of the foramen of Monro, 
so that the latter is separated from the roof (Deckplatte) by 
the whole width of the rudimentary optic thalamus. 
The foramen of Monro is further bounded above by the 
epithelial fold which forms the rudiment of the choroid plexus 
of the lateral ventricle, and which is attached to the ventral 
border of the fascia dentata (fig. 15, f. d.). 
Below, the foramen is bounded by the lamina infra-neuro- 
porica (figs. 2 and 10), which here becomes continuous with 
the corpus striatum (vide fig. 10). Posteriorly the foramen is 
bounded by the junction of the thalamic rudiment with the 
corpus striatum (fig. 11). Anteriorly the boundary is formed 
(vide the right side of fig. 15) by the fusion of the lamina 
terminalis with the ventral border of the fascia dentata. If, 
therefore, we leave out of consideration the choroid plexus of 
the lateral ventricle, it is evident that the foramen of Monro 
has no immediate relation to the Deckplatte in this brain. 
Before discussing the relation of the hemisphere to the Deck- 
platte the paraphysis must be described. 
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