MUSEUM OF COMPARATIVE ZOOLOGY. 127 
the embryo. Concerning the nature of this fold in Fundulus, I may 
say that my observations confirm, in the main, "Swirski's (80) de- 
scription for the Pike. The ectodermal fold develops independently 
of the mesoderm; the nuclei of the lower layer of the ectoderm are 
relatively large, and, like their cells, are more or less wedge-shaped in 
outline. It is worthy of note in this connection that the cuticular layer 
of the ectoderm is also peculiarly modified in the region of the ectoder- 
mal fold. The nuclei lose their characteristic flattened appearance 
(Fig. 54, nd), and become irregular in form and much enlarged. So 
great is their increase in size, that, although the cuticular layer is 
already materially thickened, still in the region of each nucleus the 
surface is carried outward by the enlarged nucleus beneath, and the 
external contour is thus thrown into numerous irregular elevations 
(Fig. 54). In Fundulus, as in the Pike, the ectodermal fold does not 
extend back along the side to the ventrals, but only through the region 
of the pectoral plates. It cannot be held that the ectodermal fold 
takes its beginning as the result of an outward growth of the meso- 
dermal cells, for the latter do not commence their outward migration 
until after the formation of a distinct lumen or fold (pli. ec'drm., 
Fig. 55). The statement that the formation of the ectodermal fold 
takes place prior to the outward migration of the mesodermal elements 
cannot be construed to mean that the modification of the ectoderm pre- 
cedes that of the mesoderm in the development of the pectoral fin ; since 
it has been shown in Fundulus (Plate VI. Figs. 40-43) that the pectoral 
plate is developed before any modification in the ectoderm. 
Hence it follows that in Fundulus the ectodermal fold does not begin 
to be formed until several days after the formation of the pectoral plate 
in the mesoderm, as already described ; and the statement of authors 
that the earliest trace in the development of the pectoral iin in Osseous 
Fishes is the formation of the ectodermal fold, does not apply to Fun- 
dulus at least, 
Nor can it be said that there is at any time in Fundulus any trace of 
a continuous longitudinal modification of the ectoderm along the side 
of the embryo, such as has been observed by Balfour in Elasmobranchs, 
and maintained by Ryder for the Cod, Stickleback, Shad, etc. I am 
convinced from my own observations upon Shark embryos, that in the 
latter, as in Fundulus, the earliest step in the development of the 
pectoral fin is not a modification of the ectoderm, as supposed by 
Balfour and accepted by Dohrn, but that the beginning must be referred 
to the proliferation of the mesoderm in the somatopleure, as I have 
