Page:Popular Science Monthly Volume 61.djvu/545

Rh vertebrates, arise locally, not, however, as blunt processes but as short longitudinal folds, with perhaps a few exceptions. The pectorals of Lepisosteus originate in the same way. Of the paired fins, the pectoral or anterior pair seems to be the first to be developed, the ventral or pelvic pair often making its appearance until after the absorption of the yolk-sac has been completed, in other cases, before that event as in Salmo and in Gambusia. The ventral undergoes less alteration of position during its evolution than the posterior pair."

In the codfish (Gadus callarias) the pectoral fin-fold "appears as a slight longitudinal elevation of the skin on either side of the body of the embryo a little way behind the auditory vesicles, and shortly after the tail of the embryo begins to bud out. At the very first, it appears to be merely a dermal fold, and in some forms, a layer of cells extends out underneath it from the sides of the body but does not ascend into it. It begins to develop as a very low fold, hardly noticeable, and as growth proceeds, its base does not expand antero-posteriorly but tends rather to become narrowed, so that it has a pedunculated form. With the progress of this process the margin of the fin-fold also becomes thinner at its distal border, and at the basal part mesodermal cells make their appearance more noticeably within the inner contour line. In some species I am quite assured that there is a mesodermal tract or plate of cells developed just behind the auditory vesicles, just outside the source of the mesodermal cells which are carried up into the pectoral fin-fold. This is developed at about the time of the closure of the blastoderm and these lateral mesodermal folds of tissue may be called the pectoral plates. The free border of the fin-fold grows out laterally and longitudinally, expanding the portion outside of the inner contour line of the fin into fan-shape. This distal thinner portion is at first without any evidence of rays; further than that there is a manifest tendency to a radial disposition of the histological elements of the fin."

The next point of interest is found in the change of position of the pectoral fin by a rotation on its base. This is associated with changes in the development of the fish itself. The ventral fin is also, in most fishes, a short horizontal fold and just above the preanal part of the median vertical fold which becomes anal, caudal and dorsal. But in the top-minnow (Gambusia), of the order Haplomi, the ventral first