Observations on the changes the ovum of the frog undergoes during the formation of the tadpole

Abstract

The ova of the Frog, when examined in the ovaria, consist of dark coloured vesicles, which acquire a gelatinous covering on entering the oviduct, and are completely formed by the time they reach the cavi­ties in which the oviducts terminate, and during their expulsion from which they receive the male influence; after this, the contents of the ovum, previously fluid, coagulate and expand, the central part being converted into brain and spinal marrow, while in the darker sub­stance of the egg the heart and other viscera are formed. The mem­brane forming the vesicles being destined to contain the embryo when it has become a tadpole, enlarges as the embryo increases, and may be said to perform the office both of the shell and its lining mem­brane in the pullet’s egg, serving as defence and allowing aëration. The black matter which lines the vesicle probably tends to the de­fence of the young animals from the too powerful influence of the solar rays, frogs’ spawn being generally deposited in exposed situ­ations. Sir Everard observes, that in the aquatic Salamander, an animal whose mode of breeding closely resembles the frog, this ni­grum pigmentum is wanting; but that that animal deposits its eggs within the twisted leaves of water plants, which afford them an equi­valent protection.