DEVELOPMENT AND LIFE-HISTORIES OF 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 (Pl. XVI. figs. 3, 6,7,* 9; Pl. XVIIL. 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 ScuuLrze pointed out, this layer in Teleosteans recalls 
the condition in the Mammalian retina (No. 144, wide 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 Horrman separates into three portions 
stratum of “tangentiale Fulcrumzellen,” a ‘‘ medialer Theil der inneren K6rnerschicht,” 
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 
an outer thin 

* Mr Cunnincuan’s figure (op. cit., pl. vi. fig. 4) appears to be, as he supposes, this species, viz., Liparis 
Montagut. 
