512 PERCIVAL BAILEY 



paraphysis. Extracts from this paper follow and will be dis- 

 cussed later. 



The lamina suprancuroporica takes a sudden bend backwards (fig. 3) 

 to form a horizontal band, which gives origin in many lowly verte- 

 brates to the plexus inferioris, and in higher animals to the plexus 

 lateralis as well, or exclusively. In the specimen under consideration, 

 however, although the plexus laterales do not actually spring from this 

 lamina, they are formed from the caudal prolongation of its lateral 



parts In describing the structures met with in a medial 



sagittal section it was mentioned that the dorsal part of the (actual) 

 anterior wall of the median cavity of the forebrain was bulged out to 

 form a large sac. The corresponding structure is well seen in the early 



embryo of Parameles In the Platypus embryo, however 



(fig. 2), a well developed choroidal fold extends from the superior 

 commissure to the lamina from which the lateral plexus arises, com- 

 pletely invaginating 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 walls of the diverticulum would seem to be merely the 

 forward continuation of the ependymal layer of the Fliigelplatten (fig. 

 15). 



As a matter of fact, the appearances in this series of coronal 

 sections are not deceptive at all, the sac being undoubtedly just 

 what it appears to be, an anterior pouch of the choroid plexus 

 of the diencephalon, the lateral walls of the pouch being actually 

 the forward continuation of the Fliigelp'atten. A glance at figures 

 1, 2 and 3 will make this quite apparent. The similarity would 

 be much more obvious if figure 3 were from a coronal instead of 

 a transverse section. In front of this pouch lies the velum 

 transversum and then the roof plate of the telencephalon (lamina 

 supraneuroporica as he calls it), from the caudal prolongations 

 of the lateral parts of which arise the lateral telencephalic plex- 

 uses. In a later article, Smith ('03) reaffirms his belief that the 

 anterior extremity of the lateral telencephalic plexus arises from 

 the roof plate. The embryo is doubtless too far advanced to 

 show the true paraphysal arch. 



Ziehen's work on Echidna is eminently unsatisfactory from the 

 standpoint of this discussion, because the sections he shows 

 invariably skip the anterior end of the lateral telencephahc plexus. 

 He has but one suggestive statement referring to this region: 



