194 G. ELLIOT SMITH. 
The Paraphysis. 
In describing the structures met with in a median sagittal 
section it was mentioned that the dorsal part of the (actual) 
anterior wall of the median cavity of the fore-brain was bulged 
out to form a large sac. The corresponding structure is well 
seen in the early embryo of Perameles (fig. 16, par.). Ven- 
trally it opens into the cavity of the “telencephalon” of His 
by a narrow neck. Posteriorly its lumen becomes suddenly 
restricted by two large bulgings from the lateral walls into 
the cavity. These bulgings are the anterior extremities of the 
optic thalami (fig. 17). On comparing the appearance of the 
Perameles brain with that of Ornithorhynchus (fig. 15) 
the general resemblance is at once apparent, the only essential 
difference being the presence of a large complicated median 
fold of the roof of the paraphysis in Platypus. ‘This is the 
anterior extremity of the ‘‘ Anlage” of the plexus choroideus 
medius—(ch. 3), the ‘‘diaplexus”’ of Wilder. In the early 
foetal brain of Perameles the median choroidal fold of the 
third ventricle is not yet developed, although that of the 
lateral ventricle (fig. 16) is well formed, so that the velum 
and roof of the paraphysis form a simple transverse band. In 
the Platypus embryo, however (fig. 2), a well-developed cho- 
roidal fold extends from the superior commissure to the 
lamina from which the lateral plexus arises, completely inva- 
ginating the paraphysis (figs. 7, 9, and 15) in the middle line. 
In Platypus the transition from optic thalamus to paraphysis 
is a very gradual one, so that in examining a series of coronal 
sections the lateral walis of the diverticulum would seem to be 
merely the forward continuation of the ependymal layer of the 
Fliigelplatten (fig. 15). This structure has attracted a con- 
siderable amount of attention since it was first noticed, and 
numerous names have been applied to it, and hypotheses 
1 «Vir die vordere Epiphyse, dem Adergeflechtknoten Goettes 
den Burckhardt in friiheren Arbeiten zuerst Plexus choroideus su- 
perior, spater Conarium genannt hatte, braucht er neuerdings mit Kupffer 
die von Selenka herrithrende Bezeichnung Parapliysis.’’—FRonizp, 
