526 



(his Fig, 4) as that of his foetal Ceratodus, agreeing as it does in every 

 essential feature with my independent reconstruction of the nerve in 

 Lepidosiren (Figure 8'). 2) Bing himself insists (p. 547) on the re- 

 markable similarity of form and identity of structure of his Ceratodus- 

 brain at stage 48 and Graham Kerr's Lepidosiren-brain at stage 35 ; 

 but my Figures 8 and 8 ' represent a reconstruction of the hemisphere 

 of Lepidosiren at stage 35, made from the identical sections which 

 Graham Kerr used in making the drawings to which Bing refers. 

 Therefore, as my reconstruction (and also Graham Kerr's) agrees 

 with Sewertzoff's, if we apply Bing's own argument to his drawing 

 (Fig. 13, p. 549) and the description of it (p. 550) his identification 

 of the parts of the forebrain in Ceratodus at stage 48 stands convicted 

 of all the errors he (unjustly, at it seems to me) attributed to Se- 



WERTZOFF. 



Before discussing the significance of that curious monstrosity — the 

 tuberculum olfactorium of the Dipneumona — it will be preferable to 

 consider the morphology of the various parts of the cerebral hemi- 

 sphere as a whole. 



After the brain has reached stage 38 (Fig, 9) its form rapidly 

 changes, When the large ventral sausage-shaped diverticulum of tuber- 

 culum olfactorium has assumed its adult form — as the result of pre- 

 cociously rapid growth — the dorsal part of the hemisphere elongates 

 and ultimately becomes longer than the tuberculum, especially in Le- 

 pidosiren, 



The olfactory bulb, which hitherto has been a flat plate forming 

 part of the roof of the hemisphere, now becomes drawn out into a 

 hollow glove-finger-like process and shifts its position so that it comes 

 to form the anterior extremity of the hemisphere. It is then placed 

 immediately above the rounded anterior end of the tuberculum 

 olfactorium. Soon a circular constriction developes at the site of its 

 connection with the rest of the hemisphere. 



Thus each cerebral hemisphere in both the Dipneumona comes to 

 assume the peculiar form, so admirably represented in the case of 

 Protopterus by Burckhahdt, of two hollow sausage- shaped chambers, 

 lying one above the other with their cavities in free communication. 

 To the anterior end of the dorsal cylinder is joined a leek-like mass 

 — the olfactory bulb and nerve ^). 



1) See Graham Kerr, op. cit. supra, Quart. Journ. Microsc. Sc, 

 PI. 26, Tigs. 8d and 9, and PL 27, Fig. 10 h and i; Burckhardt, op. 

 cit. supra, also "The Central Nervous System of Protopterus annectens", 

 Journ. of compar. Neurology, Vol. 2, PI. 13. 



i 



