MUSEUM OF COMPARATIVE ZOOLOGY. 99 
case of the median vertical fins, certain areas in these horizontal ale 
become defined as special fin regions by a visible thickening, apparently 
from the folding under of the epiblast. Thus two flattened oval pads, 
consisting of a double epiblastic fold like the double median fin-fold, are 
disengaged from the rest of the alar expanse. Before and behind this 
pair of pads the lateral membrane thins away and atrophies, while the 
special portions continue to increase in density as a pair of pectoral 
limbs.” Into these “ fin-pads,” or limb-buds, the mesoderm later enters, 
separating the two ectodermal layers of the fold. The source of the 
mesoderm of the pectoral fin is referred to “‘a mass of cells in which 
the Wolffian ducts lie and out of which they are developed” (p. 802). 
This conclusion leads the authors to the following speculations con- 
cerning the relation and evolution of the Wolffian ducts and the meso- 
derm of the pectoral fins: “If these ducts, as appears to be the case, 
arise as lateral ridges or diverticula of the somatopleure, then the meso- 
blast cells of the fins must be pronounced somatopleuric. But no 
ridge of somatopleuric cells comparable to the Wolffian ridges of higher 
forms has been recognized in fishes, and we must regard this meso- 
blast as indifferent, and forming an ‘intermediate cell-mass’ adjacent 
to the excretory system.” The observations by M’Intosh and Prince 
refer to various Bony Fishes, including Gadus morrhua, Molva vulgaris, 
Trigla gurnardus, Cottus scorpius, etc. The statement is made (note, 
p- 801) that in all forms studied at the St. Andrews Marine Station 
the pectoral fins arise out of the lateral extensions of ectoderm and 
entoderm. 
(6) Elasmobranchs, etc. — Balfour (78, pp. 101, 102), as is well known, 
was one of the first to advocate the theory that the limbs of Vertebrates 
are remnants of a continuous lateral fin. This view of the phylogeny of 
the Vertebrate limb led him to a careful study of the development of the 
paired fins in the ontogeny of several types of Elasmobranch Fishes, 
including Torpedo, Pristiurus, and Scyllium. A brief summary of Bal- 
four’s extensive and detailed account may include the following condi- 
tions in the early development of the paired fins: (1) a continuous 
lateral thickening, or ridge of ectoderm, extending from the head to 
the level of the anus ; (2) between the regions of the pectoral and pelvic 
fins this ridge of modified ectoderm may be so slight as to be observable 
only upon a careful examination of sections; (3) special developments 
of this lateral ridge — one opposite the anterior end of the segmental 
duct, the other just in front of the anus— constitute respectively the 
rudiments of the pectoral and pelvic fins; (4) the rndiments of the an- 
