MUSEUM OF COMPARATIVE ZOOLOGY, 97 
Prince (’86, p. 697) regards the pectorals of Osseous Fishes as at the 
beginning ectodermal in origin. He states that “they are differentia- 
tions of a continuous lateral expansion of epiblast, passing along each 
side of the trunk, and are formed by the folding of this epiblastic layer 
upon itself at the point where the fins appear. Each fin consists, there- 
fore, of two epiblastic lamellæ (separated by a fissure), lying flat upon 
the vitellus, and continuous with the extra-embryonic blastodermic 
membrane.” The participation of the mesoderm is presumably a sec- 
ondary process. Concerning the origin of the mesodermal cells which 
are introduced into the fin-fold, Prince says, “ They seem to be derived 
from the ‘intermediate cell-mass’ in close proximity to the Wolffian 
ducts.” 
Ziegler (’87, p. 619), in his excellent paper on the origin of the blood 
and the vascular system in the embryonal Teleost, has touched upon 
the source of the mesodermal elements of the pectoral fin, He has 
observed in the embryos of the Salmon and Pike, in stages immediately 
following the closure of the blastopore, a layer of cells lying upon the 
somatopleure just back of the gill region. This layer of cells he regards 
as the beginning of the pectoral fin. Anteriorly, it is intimately con- 
nected with the mesoderm of the head; it extends laterally in front 
of the first protovertebra, and obliquely backwards upon the somato- 
pleure, gradually losing its identity in the latter. Ziegler also states 
that the development of the pectoral fin is accompanied by the for- 
mation of a lateral longitudinal fold of the ectoderm, but does not 
give details, 
Ryder regards the modification of the ectoderm as the earliest dif- 
ferentiation in the development of the pectoral fins in Teleosts, and 
therefore the differentiation of mesoderm as a secondary process, His 
observations have been very extensive, and include representatives of 
various groups of Osseous Fishes. He states (865, p. 24) that in the 
stickleback, Apeltes quadracus, “ immediately behind the auditory vesi- 
cles, and shortly after their invagination, the rudiments of the breast 
fins appear as a pair of low longitudinal folds." In the common Shad, 
Clupea sapidissima ('86*, p. 43), the modification of the ectoderm is the 
earliest process noted in the development of the peetorals. It is im- 
plied (’82, p. 293) that in the Silver Gar, Belone longirostris, substan- 
tially the same condition exists. Many of Ryder’s observations are 
apparently based upon surface views only. He (84, p. 65) states that 
in the evolution of the pectoral fin of the Cod, Gadus morrhua, the 
first appearance is “a slight longitudinal elevation of the skin on either 
