798 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



the embryos of species with pelagic ova, e.g., Gadus morrhua, G. ceglefinus, G. mer- 

 langus, Molva vulgaris, Trigla gurnardus, and the Pleuronectidse, such median fin- 

 rays do not appear even in the late larval condition (ef, PI. X. figs. 1,4; PL XVII. 

 figs. 2, 10, 12) ; and Eyder instances other examples, some having demersal ova, e.g., 

 Alosa, Pomolobus, Cybium, Parehippus, and Idus, in which this is so (No. 141, 

 p. 518), the original transparent membranous condition of the embryonic fin persisting 

 to a late stage. Eyder adds that in Gambusia and certain Lophobranchs no embryonic 

 fin-fold is formed at all — the single dorsal fin arising later as a local dermal excrescence 

 with a core of intruding mesoblast (No. 141, p. 518). In some Cyprinoids (Idus and 

 Carassius), which also possess a single dorsal only, the continuous embryonic membrane 

 nevertheless appears. PL XVII. fig. 5, represents a young gurnard in which the two 

 dorsals and the single anal fin (of) are indicated; but the former are still continuous 

 with the tail-membrane (cf), while a remnant passes forward to the anal fin. The stage 

 figured is post-larval, and in PL XVII. fig. 7, the fins have really reached the adult 

 condition, and are completely differentiated, all trace of the continuous embryonic mem- 

 brane having disappeared. In PL XV. fig. 6, representing Cyclopterus lumpus, these 

 intermediate connections are still discernible, though the two dorsals (df) and the 

 anal are almost wholly separated. These unpaired fins have become conspicuous 

 by hypertrophy at three points, and by the atrophy of the membrane in front and 

 behind (see also fig. 5). The ventral median fin is broken up into two by the anus (a), 

 which, e.g. in Gastrosteus, has pushed its way down and terminates at the apex of an angular 

 bay marking off a pre-anal from a true anal fin (PL XV. fig. 5). Lereboullet describes such 

 a bay in the newly emerged embryo of Perca, while the body is still encircled by the con- 

 tinuous fin, " the lower edge," he says, " exhibits an indentation where the anus will appear " 

 (No. 93, p. 616). This condition differs very much, it is unnecessary to point out, from 

 that in the newly hatched embryos of the species here described. A post-larval flounder, 

 5 "8 mm. in length, which is perfectly translucent and colourless, but has lost almost every 

 embryonic trace, still retains a membranous vestige connecting the dorsal above and the 

 anal below with the caudal fin. The three fins thus connected have otherwise attained 

 all the characters seen in the adult. 



The unpaired fins in Teleosteans, therefore, do not arise as two apposed, independent 

 epiblastic plates, but as a median fold or crest. 



Prof. Humphry first broached the idea, from an examination of the adult anal fin, 

 that it might be double in its origin, i.e., a union of two lateral fins ; and he suggested 

 that the other median fins might have thus originated, and that the paired and unpaired 

 fins were alike double primitively (No. 72), a view supported by the fact that the dorsal 

 fins, in addition to their (spinal) motor nerves, are supplied by a pair of sensory nerves 

 which branch off" from the trigeminal soon after it emerges from the roof of the skull. 

 The study of their development, however, would seem to yield an opposite conclusion — 

 the median fins are single at their origin * and their bilateral structure — muscular and 



* Lericboullet's statement that the dorsal fin is double at its origin is likewise misleading (No. 93, p. 630). 



