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membranous portions of the roof of the brain are the thinned-down 
remnants of some pre-existing nervous material, and I cannot conceive 
that a non-nervous epithelial bag should be formed in lower Verte- 
brates only for the purpose of being converted into nervous material 
as the animal rose in the scale of evolution.“ 
I may perhaps be allowed to say that my conclusions were arrived 
at perfectly independently of GASKELL’s, and are based on very differ- 
ent observations, indeed, his views are merely a deduction from the 
absence of evidence, and not the result of comparative morphological 
investigations !). 
A survey of all recent work on the central nervous system of the 
fishes entitles us to say that the fore-brain in Ganoidei, Tele- 
ostei and Marsipobranchii (and my own work shows that Myxine 
forms no exception) is represented by basal and lateral structures, and 
that the homologue of the epithelial wall of the plexus choroideus of 
the third ventricle in higher forms extends over the whole roof of the 
fore- brain. 
In the Selachii, Dipnoi and Amphibia the epithelial roof 
is pushed backwards, and becomes invaginated within the cavity of 
the third ventricle by the upward and lateral growth of the basal 
ganglionic structures of the fore-brain (in their widest sense). 
Thus we have obtained one important difference between these 
two divisions of the Ichthyopsida. 
If we now turn to the urinogenital organs, and consider certain 
parts of these in the light of comparative anatomy and development, 
we shall meet with some even more striking facts. 
The Pronephros of the Ichthyopsida. 
The questions involved in the ontogenetic and ancestral histories 
of the pronephros, mesonephros etc. will demand a fuller discussion 
in a subsequent paper. I would consider the conclusions of recent 
writers EısıG, RÜCKERT, VAN WIJHE etc., not only in the light of 
their researches, but also in that of my own work on Ganoidei. 
In the present contribution it is only proposed to review some of 
the most salient facts, which recent work has brought to light, in their 
bearings on the natural grouping of the Ichthyopsida. 
Of the segmental duct (Urnierengang) nothing need here be said, 
beyond the remark that it is undoubtedly an homologous structure 
throughout the Vertebrata — that goes almost without saying. 
1) Eighteen months ago or thereabouts my opinions were known to 
Profs. Howrs and WirpERsHEIN. 
