MAR1NK TKLKOSTKAX DKYKU )I'M KXT. 85 



innermost part probably constituting a Malpighian layer. A 

 sub-epidermal space is often present, especially in those reared 

 in confinement. No true skin at first occurs beneath the 

 latter, but pigment-corpuscles appear in it, the hues in the 

 different species being described elsewhere. Beneath the 

 former, and constituting the true skin, the mesoblast extends 

 later ; in this the scales are subsequently developed and burst 

 through the epiblast. When pigment occurs over the yolk 

 it develops in the periblastic covering. In the development 

 of a pigment-spot, as a rule, a colourless corpuscle, often 

 branched, precedes it. At a late stage iridescent plates occur 

 in this layer. 



The Fins. Median Fins. 



The embryonic median fin arises as a minute fold of the 

 outer layer (epiblast) of the embryo within a day or two of the 

 closure of the blastopore. This becomes a broad membrane, 

 which increases after hatching. Its thinness and transparency 

 are remarkable. In section it consists of the two layers of the 

 epiblast and a central fissure continuous with the sub-epidermic 

 space, which is filled with a jelly-like lymph and gives passage 

 to delicate nerve-strands to the sensory papillae of the skin. 

 In this continuous fold, as F. M. Balfour said, by local hyper- 

 trophy the permanent unpaired fins arise, but atrophy of the 

 interlying membrane takes place during development. Certain 

 parts also in early stages are characterised by remarkable 

 increase of pigment. 



Shortly after hatching very fine fibrillar lines appear in 

 this continuous fin, commencing generally in the tail. They 

 form at first granular tracts indefinite in outline, and un- 

 connected with the axial skeleton. They are usually termed, 

 after Ryder, embryonic fin-rays. The mesoblast at a later 

 stage extends between the layers of the fin, and thus it is this 

 layer which gives rise to the true rays, which appear in the cod 

 as three dorsal and two anal fins. Each ray at first forms a 

 slight opacity extending towards the free margin, and the 

 intermediate membrane disappears by absorption. The de- 

 velopment of a mesoblastic granular thickening, a short distance 



