DEVELOPMENT AND LIFE-HISTORIES OP TELEOSTEAN FISHES. 767 



distinct. During the first week after hatching, the internal molecular layer undergoes 

 great development, and rapidly becomes a thick and bold stratum separating the external 

 granular layer from the internal granular layer. The external molecular stratum is 

 slowly differentiated on the inner surface of the columnar layer, while the inner 

 granular layer shows, though very obscurely, a separation into an inner and an outer 

 portion. The pigment of the choroid is much more abundant than before, and in the 

 living embryo gives to the eyes a dense appearance, so that the minute transparent 

 fish can usually be discerned in the tanks of the laboratory by the two large dark eyes, 

 which form a most prominent feature (PI. XVI. figs. 3, 6, 7* 9 ; PI. XVIII. figs. 1, 2). The 

 structure of the retina exhibits little further change during the later larval stages, but in 

 the post-larval conditions other features appear, which need not be noticed in detail by 

 us, as Dr Marcus Gunn has specially occupied himself with this subject. Thus in a 

 young flounder, still transparent and colourless, the pigment-layer is greatly increased in 

 thickness, and it sends prolongations into the bacillary layer. The cylindrical rods 

 form a very distinct stratum, while the flask-shaped cones are well defined, and present 

 a contrast to the corresponding layer in Amphibians, which have a very insignificant 

 stratum of cones. Indeed, as Max Schultze pointed out, this layer in Teleosteans recalls 

 the condition in the Mammalian retina (No. 144, vide sect. iv. of his paper). The 

 double disposition (twin-cones) in the adult eye of osseous fishes has not yet been 

 assumed, so far as can be made out. The striking coloured globules so prominently seen 

 in this layer in Batrachians, birds, and some reptiles are absent, nor do they at any 

 subsequent stage appear to be developed. That Teleosteans should have a layer of rods 

 and cones so early and so well developed, whereas in Selachians (and cartilaginous fishes 

 generally, it is said) no cones can be made out, is a remarkable circumstance. Bats, 

 hedgehogs, and other nocturnal forms amongst Mammals, are destitute of cones. 



The limitans externa in the post-larval stages is a very delicate lamina ; but it is 

 well defined. The external granular stratum now consists of several layers of large 

 cells separated from the inner granular layer by a comparatively broad external mole- 

 cular layer. 



The inner granular layer itself Hoffman separates into three portions — an outer thin 

 stratum of "tangentiale Fulcrumzellen," a " medialer Theil der inneren Kornerschicht," 

 and a " lateraler Theil " of the same layer. In the flounder, as well as in such forms as 

 Cottus and Cyclopterus, only the outer "tangential " cells can be distinguished from the 

 remaining elements of the inner granular layer, which form a very thick band. Internal 

 to the last-named layer is the internal molecular stratum, anterior to which the 

 ganglionic layer can be distinguished. The internal molecular layer Hulke describes as 

 including a large quantity of connective tissue, in the midst of the fibres of which are 

 large branched corpuscles of very considerable dimensions (No. 71, p. 247), but in com- 

 paratively late post-larval stages no trace of these structures can be made out. The 



* Mr Cunningham's figure (op. cit., pi. vi. fig. 4) appears to be, as he supposes, this species, viz., Liparis 

 Montagui. 



