DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 801 
mesoblast apparently extends into it (a/, Pl. II. figs. 11-13; Pl. IV. figs. 4, 10), 
though this layer is ill-defined laterally at this stage. A pair of lateral horizontal ale 
(a/), indeed, stretch along the whole trunk—from the pectoral to the post-mesenteric 
region. It is in reality the elongated and narrowed blastodermic scutum (Pl. XXVIII. 
fig. 5), and extends in front and behind the two points mentioned, though it is there 
thinner and hardly distinguishable. In Pl. III. fig. 19, such a pair of lateral horizontal 
fin-expansions are present extending from the trunk-region proper, and their limits 
are very definite when viewed from above. Just as in the case of the median vertical 
fins, certain areas in these horizontal alze become defined, as special fin-regions by a 
visible thickening, apparently from the folding under of the epiblast. Thus two flattened 
oval pads consisting of a double epiblastic fold like the double median fin-fold, are 
disengaged from the rest of the alar expanse. Before and behind this pair of pads the 
lateral membrane thins away and atrophies, while the special portions continue to increase 
in density as a pair of pectoral limbs (pf, Pl. V. figs. 6,9; Pl. XIV. fig. 1). LereBouLLer 
apparently did not notice that the pectoral fins emerge from the lengthy lateral mem- 
brane or alar expanse on each side, and speaks of a gradual accumulation of cells from 
the inferior lateral portions of the trunk as a pair of tubercular processes protruding 
some distance behind the ears. In Perca he found that these fin-pads became detached 
on the seventh day (No. 95, p. 10; No. 93, p. 583). The increasing density of the fin- 
pads is due to the entrance of mesoblast into the interstice, separating the upper from 
the lower epiblastic lamella. This mesoblast spreads out radially, but does not reach 
quite to the distal margin, and the peripheral portion remains more transparent, though 
the epiblastic cells which solely constitute it become columnar, and form a thickened 
ridge from which the fin-rays doubtless subsequently develop centripetally.* Such a 
mode of development as that above sketched has theoretical bearings of considerable 
interest. These were briefly treated in a former note (vide No. 124a, p. 697), and need 
not be discussed in this place further than to point out that the fin develops as a 
horizontal ridge, in accordance with Barour’s theory of a primitive horizontal lateral 
fin, and that it is independent of and prior to the formation of a girdle-rudiment. Prof. 
CLELAND, in a paper on the Limbs of Vertebrates (No. 40), emphasised this latter point, 
and further showed that a limb involves two distinct elements—a radiation (7.e., an 
appendix) and an arch, which is not a radiation, but a cincture, which always circles more 
or less round the body, and may be complete above or below. Prof. CLeLanp further 

stated that neither appendage nor limb-arch is the property of one particular segment,— 
their position being variable and their nervous supply multisegmental,—points which are 
* Kinestey and Cony, in the cunner (Zautogolabrus adspersus, Gill), and other authors in various forms, have 
recognised only the lateral fins when they were defined as tubercular pads. The observers named speak of these fins 
as only developed when the embryo is ready to emerge—the tail being free and the capsule loosely surrounding the 
fish (vide No. 78, fig. 51, pl. xvi.). No trace of a continuous lateral fold could be seen, the fins protruding as simple 
outgrowths (p. 210). The extension of the thickened epiblast and hypoblast laterally is, however, a feature common 
to all Teleostean embryos, and a portion of this becomes defined in all the forms studied at St Andrews, and out of 
this defined epiblastic fold the pectoral fins arise. 
VOL. XXXV. PART III. (NO. 19). 6K 
