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 (Pl. XVIL. 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 Pl. XIII. 
figs. 2, 6a; Pl. 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 (Pl. VII. figs. 3, 4). or even so late as the seventeenth day 
(Pl. XI. fig. 14). LeresouLLer noticed similar indications in the still persisting 
membrane of the embryo of Pevca 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). Ryper 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 (PI. 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 ewryophthalmus. 
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 
* Ryper 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. 
