DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 801 



mesoblast apparently extends into it (al, PI. III. figs. 11-13; PI. IV. figs. 4, 10), 

 though this layer is ill-defined laterally at this stage. A pair of lateral horizontal alee 

 (al), 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 alee 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). Lereboullet 

 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 centripetaily.* 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 Balfour'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 (i.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. Cleland 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 



* Kingsley and Conn, in the cunner (Tautogolabrus 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, pi. 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). 6 K 



