800 PROFESSOR W. C. M‘SINTOSH AND MR E. E. PRINCE ON 
which appears in certain parts of the axial and appendicular skeleton (e.g., vertebral 
bodies and pectoral arch). Most probably they are of a resistent horny (?) nature, and 
they are developed at first in the distal or mid-part of the fin-membrane, approaching, 
as before pointed out, the trunk by the growth of the proximal end of each ray, “ their 
development being in conformity with M. Serres’ law of centripetal formation” (No. 93, 
p. 634; also vide Serres’ Principes dorganogénie, Paris, 1842, p. 212). As the rays thus 
develop, the aboral end of the cellular notochord (nc) curves upward (Pl. XVIII. fig. 3), the 
upper lobe (opisthure of RyprEr) diminishes, while a new and larger lobe expands on the 
ventral side of the chorda. A notch, however, separates this new growth from the lower 
lobe of the primary protocercal tail (PI. XIX. fig. 4).* AGassiz describes this development 
of the secondary caudal membrane as an atrophy of the upper lobe, and a rapid 
development of the lower lobe which becomes bifid. The lower lobe does not really 
become bifid, but a new lower or rather anterior ventral lobe grows out, and by its 
rapid development leaves a notch separating it from the primary lower lobe. The two 
original lobes of the protocercal tail are gradually pushed further up and almost entirely 
disappear, the tail of the adult being for the most part a wholly new growth on the 
ventral side of the notochord, and slightly anterior to its termination (compare figs. 3 and 
5, Pl. XVIIL). The stages of this atrophy of the primary fin-lobes and the growth of the 
secondary tail-fin, mainly as a new product, can be seen by comparing Pl. XVII. fig. 3, 
which shows the original protocercal outline, with fig. 6 on Pl. XV., in which the 
secondary tail-fin is formed as a large ventral lobe supplanting the primary tail. 
In fig. 5, Pl. XVIL, the new tail-fin has completely taken the place of the primary 
membrane. Pl XVIIL, figs. 3, 4, 5, and 7, show these stages well. The embryonic tail with 
its dermal rays is transitory, and the permanent tail with its hypural elements (Pl. XV. 
fic. 3) belongs to a stage which is post-larval. LEREBOULLET says the materials out of which 
these later skeletal elements are developed are furnished by a rich caudal plexus of blood- 
vessels. This complex vascular development, he says, ‘precedes and announces the 
formation of the tail,” and it consists of a system of elongated loops in the pike, perch, 
trout, and roach (No. 95, p. 26). No such subnotochordal terminal plexus is formed in the 
Gadoid and other forms studied at St Andrews. Thus the gurnard, even at so advanced 
a larval stage as Pl. XVII. fig. 5, shows no such network; yet the hypural plate is well 
developed and the fin rays fully defined. 
The Paired Fins.—When the embryo is first outlined in the blastoderm, an alar 
expansion stretches away on each side of the trunk of the young fish. This expansion 
consists of epiblast and hypoblast resting upon the stratum of periblast below. No 
* Ryper (“ Evolut. of Fins of Fishes,” Report of Com. Fish and Fisheries for 1884-1886) states that there is evidence 
of the degeneration of the caudal region, as in Chimera and Stylephorus there is a permanent archicereal opisthure, a large 
temporary one in Lepidosteus; and, moreover, there is the evidence of the concrescence of the hypural pieces; the 
ventrally diplacanthous and even triplacanthous caudal vertebra, or their coalesced representative, the urostyle ; the 
existence of hypaxial opisthural elements ; the abortion of the epaxial spines of the caudal vertebra ; and finally, the 
abortion or extreme modification of the last muscular somites of the caudal region. Ryper (op. ectt., from an examina- 
tion of the eel) holds that the hypurals are partly heemal and partly interspinous. 
+ See Lorz on “Tail of Salmon,” &c., Zeitschr. f. wiss. Zool., 1864, p. 260. 
+ 
