Two Embryos from One Egg

Experimental embryology, or, as it is better known, developmental mechanics, started on its class-conscious career about thirty- five years ago, and its infancy is remembered chiefly for two events. The first of these was Roux's discovery (1888) that a half embryo develops from one of the first two blastomeres (cells) of the frog's egg when the other blastomere is injured but present. The second was Driesch's discovery (1891) that a whole embryo develops from each of the first two blastomeres of the sea urchin's egg when they are separated from each other. The latter discovery was followed by a study of the eggs of other animals and led to the apparent paradox that in some animals a whole embryo of half size develops, and in other animals a half embryo develops from each of the isolated blastomeres. These results were idealized as two contrasting schemes of development, and far-reaching philosophical speculations were based upon them. Echoes of the controversy that followed may be heard to-day, although interest in them appears to be on the wane. Novel as were the results themselves, it must be admitted that they have not led to the solution of any of the fundamental problems of development. They served, however, a useful end in calling attention once more-for His had attempted to do this already in 1874 -- to the scientific problems of embryology from which students of embryology had been diverted since 1859 by over-attention to the historical side of the subject. It is not my intention to enter here into the conflict of opinion that arose between vitalist and mechanist, for even were there any profit in doing so, which I doubt, my object is to review the facts from a different angle rather than to raise philosophical and controversial questions