394 Bashford Dean Memorial Volume 



stalk is twofold: it prevents the eye from sinking too far into the socket, and it supplies 

 a more lateral basis for the origin of the recti muscles. The lateral rectus consists of two 



parts which have separate origins and insertions, although they are otherwise united by 

 strong "strands of muscle nbers. One di\-ision or this muscle takes origin from the outer 

 part of the optic stalk, while its insertion is on the posterior surface of the eyeball. This 

 is the normal insertion for an undivided rectus lateralis. The other division is said by 

 Hawkes to be twice as large, though in Nishi's figures (reproduced here as Text-figures 

 66 and 67) it appears sHghtly smaller than it does in Hawkes" figures. Its origin is from 

 the cranium as well as along the proximal portion of the optic stalk. The insertion is on 

 the dorsal side or the eyeball, somewhat more external than that of the rectus superior 

 which it partly overlaps. From the positions of its origin and insertion, this division 

 must be considered as a secondary' or deri\-ative portion or the primitive rectus lateraUs. 

 This secondary' muscle was probably spHt off from a rj-'pical rectus laterahs to aid the 

 superior rectus and the superior obhque in tilting the eye upward. The recti superior, 

 medialis (intemus) and inferior are all attached to the top of the optic stalk, just below its 

 flattened head. 



THE APPESTDICXX.\R MUSCLES 



From embryological studies on certain elasmobranchs and primitive teleostomes it 

 is clear that, in these fishes, buds from the myotomes grow into the embryonic fins and 

 there break down into mesenchyme which is the source of the fin muscle : as in Spinax 

 Braus, 1899 -^ ScvUfuvn (Goodrich, 19061; Cestracion (Osbum, 1907J; Acar.thias (E. 

 Xliiller, 1911,i; in Amia and Lepidosteus (Schmalhausen, 1912). Thus the muscles that 

 move the fins are metameric in origin: this appHes to both paired and unpaired fins. 

 Some features of this developmental history have been interpreted in terms of the fin-fold 

 theory of the origin of paired fins. Concerning this matter, Daniel (1934, p. 110) says: 



It is evident that the number of segments that take part in the formation of buds for the 

 pectoral fin is fewer in the sharks than in the rays. This fact is clear when we consider two 

 types like Miisteliis and Torpedo, in the former of which the fin is relatively narrow and in 

 the latter is of great extent. According to Maurer (1912), in the embryo of Mustelus only 

 10 s^ments contribute to the formation of the musculature of the pectoral fin; while in 

 Torpedo there are 26 such s^ments. 



The further course of the development of these buds in two forms like the above has been 

 studied in great detail because of the bearing which such development has on the lateral 

 fin-fold theory. That, in a type like Mustelus, segments (myotomes) anterior to the pectoral 

 fin and between the pectoral and the pelvic fins form buds which atrophy without entering the 

 fin, is taken by those who accept the lateral fin-fold theory to mean that the fin previously 

 had a much greater anteroposterior extent than at present; and it is hence in agreement with 

 what would be expected from that theory. 



In common vi-ith the nbtidanids, Chlamydoselachus seems to afford tavorable material 

 for the study of the origin and development of the fin muscles, but so far as I am aware, 

 such studies have never been made on these forms. 



Of the fins of Chlamydoselachus, only the pehncs of the male have received attention 

 ■^ith respect to their musculature. The muscles oi these fins have been described in 



