800 PROFESSOR W. C. M'INTOSH 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 d'organogenie, 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 Ryder) 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 (PL 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 low r er 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 XVIII.). 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 wdiich the 

 secondary tail-fin is formed as a large ventral lobe supplanting the primary tail. 

 In fig. 5, PL XVII. , the new tail-fin has completely taken the place of the primary 

 membrane. PL XVIII., 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. 

 fig. 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.f 



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 



* Ryder (" 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 Chimcera and Stylephorus there is a permanent archicercal 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 vertebra3 ; and finally, the 

 abortion or extreme modification of the last muscular somites of the caudal region. Ryder [op. cit., from an examina- 

 tion of ih I) lmlds that the hypurals are partly haemal and partly interspinous. 



+ See Lotz on "Tail of Salmon," &c, Zeitschr. f. wiss. Zool., 1864, p. 260. 



