DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 797 



same plate, fig. 11. A similar dorsal and ventral arrangement of caudal pigment-spots 

 occurs in the advanced embryo of the ling (PI. XVII. fig. 10), black pigment-spots 

 diverging upward and downward from the caudal trunk in a characteristic manner. In 

 this way the sites, so to speak, of the future median fins are indicated by radiate 

 coloration before the continuity of the embryonic membrane (ef) is to any appreciable 

 extent destroyed. Later, however, the developing fin-rays (embryonic) are more clearly 

 indicated by granular striations which pass across the membrane (vide PI. XTII. 

 figs. 2, 6a ; PI. XV. figs. 4, 5), still very thin and transparent (though a fine reticulation 

 of a superficial character often appears in it), no mesoblast having as yet insinuated 

 itself into the interlamellar fissure, as shown in a section of the haddock on the 

 third day after hatching (PI. VII. figs. 3, 4) ; or even so late as the seventeenth day 

 (PL XI. fig. 14). Lereboullet noticed similar indications in the still persisting 

 membrane of the embryo of Perca when twelve to fifteen days old. He describes along 

 its whole length small irregular transparent structures like oil-tracts, and he found that 

 they accumulate where the permanent fins will be developed (No. 93, p. 640). These 

 are either the homologues of the pigment-corpuscles mentioned above, or aggregations of 

 the external reticulation. Later, he says, he noticed these disappear in Leuciscus eury- 

 ophthalmus as if by absorption, and striations inclined in a backward direction take their 

 place. They form successive pairs, the rudimentary rays, in fact, of the unpaired fins, 

 which he remarks are double at the time of origin (p. 640). Eyder speaks of the 

 mesoblast as entering the fold at an early stage (No. 114, p. 517),* but this does not 

 apply to many forms, for a section through an advanced embryo of the haddock, as just 

 mentioned (PL XI. fig. 14), still shows a mere epiblastic fold (ep) little altered from its 

 primitive condition. While the membrane still remains thin and translucent, ray-like 

 thickenings are frequent — apparently aggregations of a horny or chitinous nature, usually 

 regarded as epiblastic thickenings, which develop, as Lereboullet observed, centri- 

 petally, and grow towards the trunk (No. 93, p. 637). He describes them as transparent 

 strips, distant from, but directed towards the body, and appearing simultaneously in the 

 three parts which subsequently form the three vertical fins in Leuciscus euryophthalmus. 

 These rays Lereboullet describes as formed by a " condensation of a plastic material 

 without any grouping of cells," and he regards them as connected with the vertebral 

 column below from which they are separated, subsequently, by the interspinous bones 

 (p. 630). In reality, however, the early rays are merely dermal thickenings, and appear at 

 first as narrow granular tracts indefinite in outline, and extending dorsally and ventrally, 

 and therefore unconnected with the axial skeleton below. Lereboullet's view applies 

 to the dense permanent rays which develop in the post-larval stages, for these rods 

 are paired, and arise under the epiblast — beneath the pigment, which appears in the 

 Malpighian layer of the ectoderm, and are most probably aggregations of mesoblastic cells 

 which grow up into the median fin-fold from the axial (skeletal) mesoblast below. In 



* Ryder now holds that even the embryonic fin-rays are mesoblastic {Rep. U.S. Comm. Fish and Fisheries, 1884). 

 As fast as they appear, they are preceded or accompanied by outgrowths of mesoblastic cells. 



