Cr 
Raymond C. Osburn 1 
are to be regarded as most primitive. In this respect the writer has been 
most fortunate in having at his disposal a very complete series of embryos 
of the cestraciont shark Heterodontus japonicus Duméril, and a number 
of embryos of the Notidanid shark Chlamydoselachus anguineus Garman, 
85-86. ‘Together with this material I have had for comparison many 
stages of Spinax, Mustelus, and Torpedo. Professor Bashford Dean has 
very generously placed this valuable material at my disposal, and I am 
indebted to him also for many kind suggestions and criticisms. 
Certain observations have been made by various students of the fins 
and corroborated to such an extent that they may be considered as points 
definitely settled in selachian embryology, especially since they are not 
disputed by the followers of either the gill-arch or the fin-fold theory. 
Such are: 
I. Each fin at its inception consists of a longitudinal fold of the epi- 
dermis. This has been observed by all students of shark embryology from 
Balfour, 78, down to the present. It may be added that the same obser- 
vation holds almost universally among fishes, the few exceptions being 
among T'eleosts whose limb structures we have reason to believe are 
highly specialized. 
IJ. This fold of the epidermis is soon invaded by mesenchyme cells 
which establish a dense plate or mass of cells (mesenchyme plate or 
mesenchyme thickening) lying partly within the epidermal fold and 
partly within the body-wall. (Balfour, 78, 81; Dohrn, 84; P. Mayer, 
86; Moller, 93; Rabl, 97, or; Braus, o4b, and others.) So much at least 
is beyond question. The exact origin and fate of the mesenchyme plate 
will be discussed later. 
III. The fin-fold with its mesenchyme plate is next provided with 
segmentally arranged muscle-buds from the dorsal and ventral ends of 
the neighboring myotomes. This ingrowth in the paired fins is similar 
to that in the median fins. (Balfour, 78, 81; Dohrn, 84; P. Mayer, 86; 
Mollier, 93; Rabl, 97, or; Braus, 98, 04a; Goodrich, 06, and others.) 
IV. In the embryonic fin, whether paired or unpaired, there are abort- 
ive muscle-buds which degenerate without entering the fin. These are 
found in the posterior as well as in the anterior margin of the fin. In 
the paired fins these rudiments have been traced by Dohrn, 84; Mollier, 
93; Braus, 98; Rabl, or, and Goodrich, 06, and in the median fins by 
Dohrn, 84; Paul Mayer, 86, and Goodrich, 06. 
V. The most anterior and most posterior muscle-buds which enter the 
1Throughout this paper Heterodontus is referred to as Cestracion, the 
older name by which the genus is usually known among morphologists. 
