MUSEUM OF COMPARATIVE ZOOLOGY. 129 
the pectoral fins. Their assumption that the pectoral fin begins its dif- 
ferentiation in a stage as early, for example, as that shown in Figure 5 
(Plate I.) for Fundulus, meets with serious obstacles. It presumes 
a recognition of the pectoral fin prior to the complete differentiation 
of the primitive germinal layers themselves, and hence refers the begin- 
ning of the pectoral fin to a structure (an “alar expansion” which 
“consists of epiblast and hypoblast”) which evolves the entoderm or 
hypoblast. But in what sense the entoderm is identified with the 
beginning of the pectoral fin they have not explained. 
It is evident that a view of the blastoderm seen from above (Plate I. 
Fig. 1) presents such lateral expansions of the primitive layers as 
M’Intosh and Prince have designated ale. These expansions not only 
extend along the sides of the embryonic axis, but are also continuous in 
front of the embryonic swelling ; to draw any analogy, however, between 
this blastodermic rim and the pectoral fin, or to regard this condition of 
the primitive layers as representing in any sense the pectoral or paired 
fins, seems to me fanciful and unwarranted. At a later stage (Plate I. 
Fig. 3), in which the somatopleure has undergone a thickening in the 
pectoral region, or still later, when the pectoral plate is formed, it is 
true, as described by these authors, that a parietal (lateral) thickening in 
the pectoral region may be observed when the embryo is examined from 
above in surface view or by transmitted light; but this appearance is 
at first due entirely to a modification of the somatopleure, and at no 
time are the other layers — splanchnopleure and entoderm — concerned. 
This fact can be readily demonstrated by serial sections from the proper 
stages of the embryo ; hence I believe that the interpretations of these 
authors upon the origin of the pectoral fins are misleading, and that 
their views are untenable. 
Ziegler maintains that the thickening of the somatopleure is accom- 
panied by the upheaval of the ectoderm, and evidently does not regard 
the ectodermal fold as the first step in the development of the pectoral 
fin in the embryo of Elasmobranchs. In his studies of the Salmon 
and Pike he reaches substantially the same conclusion, but gives no 
further details concerning the formation of the lateral longitudinal fold 
of ectoderm. The facts upon which his statements are based may be 
identical with the conditions in Fundulus, but, not being specially inter- 
ested in the ectoderm, he may have omitted a more exact account of 
its modification; at any rate, it cannot be affirmed that in Fundulus 
the proliferation or thickening of the somatopleure is at first accom- 
panied by an upheayal or any other modification of the ectoderm, 
