LARVJE, ETC., OF OPHIOCEPHALUS STRIATUS. 
113 
On the fourth day the larvae have attained a length of 6*75 mm. 
They are now leaving the surface and swimming freely at all levels; 
bright yellow spots appear over the eyes. The hinder extremity of 
the notochord is still straight, but the embryonic fin has exchanged 
the vacuolated structure of the preceding days for a fine radiate 
striation. The sides of the body are free from pigment, and are 
consequently traversed by a pale longitudinal band parallel with 
the notochord. There is also a more or less interrupted pale band in 
the middle dorsal line of the fore-body, in front of the embryonic fin. 
There is a faint massing of embryonic tissue below the hinder end of 
the notochord, a little before its extremity. This is the primordium 
of the caudal fin-ray system. 
The fifth day shows no increase of length. The caudal pigment 
is increasing, though still diffuse, and the caudal primordium is 
becoming denser. On the sixth day the length is found to exceed 
7 mm., and there is a further slight concentration of pigment and 
embryonic tissue. 
On the seventh day, still with a length of about 7 mm., we find 
the first traces of the basal cartilages of the caudal rays, situated 
below the free straight end of the notochord at the point where the 
myotomes or muscular segments cease. The rudiments of three 
cartilages can be made out, but the caudal pigment, which has a 
peculiar relation to the formation of the fin-rays, is still diffuse. 
The pigment in the ventral portion of the embryonic fin is now 
beginning to predominate over that in the dorsal portion of 
the fin. 
In the next three days I observed no increase in length, but the 
caudal pigment below the end of the notochord is tending to lie in 
radial streaks, marking the position of the future caudal rays ; and 
the caudal capillary system makes its appearance. 
On the twelfth day (Fig. 11) the end of the notochord begins to 
bend up, and the caudal rays begin to show; there is still no further 
increase in length; on the contrary, as the tissues and cavities of the 
body begin to expand, there is a very slight decrease in length to be 
noted, from about 7 mm. to about 6*75 mm., this length being 
maintained to the end of the fifteenth day. Incidentally I noted 
the fact that the twelfth day larvae were not rising to the surface 
of the water to take the air. Up to this time the body of the larva 
has been colourless, except for the black pigment. On the fifteenth 
day some of the more advanced larvae display a pronounced yellow 
ground colour associated with a further condensation of the black 
cells. The contour of the caudal fin at this period is shown in 
Fig. 11. 
There is now a gap in my observations until the twenty-fifth day, 
the fry having been kept meanwhile within a small enclosure in a glass 
tank. The embryonic median fins are still continuous (Fig. 12) ; 
the ventral portion of the embryonic fin, from which the anal fin 
