Ou the larval developmeiit of Aiuia calva. 665 



which are clearly interpretable as specialized. Biit oii the other band 

 it should be noted that the brain of Acipenser is in some of its 

 features niore specialized tban that of Anna, e.g., in the high degree 

 of diti'erentiation of the cerebelluni, of the coniniissures, of the vehmi 

 transversuin. The relation of the brain type of Amia to that of the 

 Teleost, however, is clearly to be traced, the elaborately specialized 

 brain parts of the latter readily reduciug to the simple conditions of 

 Amia. This relationship may best be understood by comparison of 

 the brain shown in Fig. Q \\\i\\ that of Fig. R, a sagittal section of 

 a uewly hatched trout, Salmo fario. Similar parts are clearly 

 recognizable, but the divergences which the structures of the bony 

 fish have imdergone in the line of differentiation are most noteworthy. 

 Broad changes have befallen the mid- and hind-brain, and the region 

 of the infundibulum : in the mid-brain the lumen has become notably 

 reduced, its roof greatly enlarged, thick walled, massive, its floor con- 

 voluted in adaptation to its greater size: of the hind-brain the floor 

 is heavily thickened, the cerebellum large and convoluted: the infun- 

 dibulum is also deeply convoluted, and the sacculus ventralis is 

 relatively enlarged. It is, accordingly, evident that the brain of the 

 bony fish has increased enormously in size hindward of the chiasma. 

 In front of the chiasma, on the other band, its conditions have come 

 to differ but little from those of Amia; the anterior flexure of the 

 brain has become slightly more marked, the lobus now appearing as 

 if in the middle of the floor of the epencephalon; the anterior thicken- 

 ing of the roof of the latter is no longer prominent, and its epen- 

 dymal portion, if anything, smaller^ iu extent; the flattened roof of 

 the parencephalon and the reduced depth of the velum transversum 

 are finally to be noted as among the minor differences. 



The foregoing comparison of the brains of Amia and a bony fish 

 is not without a distinct phylogenetic interest: for it has shown that 

 the transition between the Ganoi'd and the Teleost in these structural 

 regards is by no means as broad as has been generally believed. 

 That the brain arises as a solid ectodermal thickening instead of as 

 a tubulär organ formed in the beginning by the overgrowth of the 

 medullary folds, is a feature common as well to some of the Ganoids 

 (Lepidosteus and Amin) as to the Teleosts. Nor can any distinction 

 be grounded on the epithelial character of the roof of the Teleostean 



1) In subsequent stages the ependymal portion becomes relatively 

 larger. 



