96 BULLETIN OF THE 
Oellacher says nothing concerning the histological character of these 
structures, and gives no drawings. He makes no mention of the modi- 
fication of the ectoderm in the region of the pectoral fin, and believes 
the ectodermal fold to be absent in these fins ; but he observed it in case 
of the pelvic and dorsal fins, and upon this difference he raises the ques- 
tion as to the different morphological signification of the pectoral and 
pelvic fins from the standpoint of their genesis. Oellacher was the first 
observer to trace the origin of the paired fins from the mesoderm, but 
it is evident that he failed to see the primary source of the mesoderm 
of the limb-bud, and did not understand the secondary relation between 
the pectoral plate and the protovertebra. 
’Swirski (80, p. 15) has studied the limb-buds of the young fry of 
the Pike, and gives an account of their later structure and the develop- 
ment of the rudimentary skeletal parts. His description of the appear- 
ance of the mesoderm in cross sections of the pectoral fin of a young 
Pike, a day old, substantially agrees with that given by Oellacher (79) 
for the Brook Trout. He also describes briefly the cells of the ectoder- 
mal fold, stating that the ectoderm in the region of the fold consists of 
beautiful large, cubical cells, with fine-grained nuclei and dark-grained 
protoplasm.  "Swirski notes that the ectodermal fold develops indepen- 
dently of the mesoderm, and is not continuous along the side to the 
region of the ventrals ; that it is present in the case of the pectoral 
fins is a fact which Oellacher had overlooked. ’Swirski does not, how- 
ever, note any observations upon the earlier development of the limb- 
bud, or upon the source of its mesodermal elements, and regards the 
ectodermal fold as the beginning of the pectoral fin. 
Emery (83, p. 338) first directed attention to the homogeneous stra- 
tum developed in the fin-fold of embryos of Bony Fishes. He studied 
the larve of Fierasfer, Belone acus, and Lophius, and regards the sub- 
stance between the ectodermal layers of the fin-fold as mesenchymatio ; 
this substance may originate as a secretion partly by ectoderm and 
partly by mesoderm ; and he believes that it may be comparable to the 
gelatinous substance of the umbrella of Medusæ. His observations 
were however made upon the folds of the unpaired fins. 
Kingsley and Conn (’83, p. 210) did not see the earlier stages in the 
development of the pectoral fins in the Cunner, Ctenolabrus ecruleus. 
The earliest condition noted by them was that of the ectodermal fin- 
fold just prior to the time of hatching. They say, “ The fins, which 
are first seen in an egg about as far advanced as Figure 51, arise in 
the Cunner as a simple outgrowth, and not as a continuous lateral fold, 
as is found in many forms.” 
