Nitrogen-Excretion in Selachian Ontogeny

ABSTRACT The investigations to be described in this paper were originally undertaken in order to throw light on the utilisation of protein as a source of energy by the developing selachian embryo. It has been shown that in the case of the chick (Needham (24), Fiske and Boyden (14)) and of the frog (Bialascewicz and Mincovna (4)) there exists a peak in the intensity of protein breakdown during embryonic life, on each side of which the catabolism of these substances is much diminished. In the case of the mammal (cow and sheep, Lindsay (23) and others) the descending limb of the curve has alone been observed. Our intention was to extend these investigations to a selachian egg, and we obtained several interesting results, although special difficulties were encountered, e.g. (a) the lack of any good series of weighings of selachian embryos, and (b) the remarkably high concentration of urea in elasmobranch tissues, which makes it difficult to regard it wholly as a waste-product.