98 : BULLETIN OF THE 
side of the body of the embryo, a little way behind the auditory vesi- 
cles. . . . At the very first, it appears to be merely a dermal fold, and, 
in some forms, a layer of cells extends out underneath it from the sides 
of the body, but does not ascend into it.” He adds: “In some species 
I am quite well assured that there is at an early period a mesodermal 
tract or plate of cells developed just behind the auditory vesicle, just 
outside the muscle-plates of this region, on either side, which may be 
regarded as the source of the mesodermal cells which are carried up into 
the pectoral fin-fold. This is developed at the time of the closure of 
the blastoderm, and these lateral mesodermal tracts may be called the 
pectoral plates.” In later stages of development, Ryder observes that 
mesodermal cells make their appearance at the base of the ectodermal 
fin-fold, and enter the basal part of this fold. Referring to the Carp, 
Gambusia patruelis, he (’85, p. 153) states that the muscles of the 
pectoral fins “are probably derived, as in Elasmobranchs, from buds 
given off by the muscular segments above the rudiment of the girdle” ; 
but the evidence from which this inference is drawn he does not state. 
Ryder (’86, pp. 1010, 1018) has studied the development of the fin- 
rays in the lateral ectodermal fin-fold, and has written upon theoretical 
considerations concerning the evolution of the fins in fishes. 
M’Intosh and E. E. Prince (’90, pp. 800-803) have assumed that the 
paired fins in Bony Fishes are of ectodermal origin, and begin as hori- 
zontal ridges, in accordance with Balfour’s theory of a primitive lateral 
fin. They regard the lateral margin of the blastoderm, at a stage in 
which the blastopore is not yet closed and the embryonic thickening has 
merely begun, as the first trace in the development of the pectoral fins. 
This lateral margin of the blastodermic swelling is described as an “alar 
expansion” along each side of the embryo, and “consists of epiblast 
and hypoblast resting upon the stratum of periblast below” (p. 800). 
From these lateral wing-like expansions of ectoderm and entoderm, 
between which apparently no mesoderm extends, the fins are developed. 
The views of these authors may be more fully comprehended from the 
following quotation (p. 801): “A pair of lateral, horizontal ale (a/.), 
indeed, stretch along the whole trunk, —from the pectoral to the post- 
mesenteric region. It is in reality the elongated and narrowed blasto- 
dermic scutum (Plate XXVIII. Fig. 5), and extends in front and behind 
the two points mentioned, though it is there thinner and hardly distin- 
guishable. In Plate III. Fig. 19, such a pair of lateral horizontal fin 
expansions are present, extending from the trunk region proper, and 
their limits are very definite when viewed from above. Just as in the 
