THE CAUSES OF DEVELOPMENT. These are diffi cult to discover. In the eighteenth century the idea prevailed that development consisted merely in the unfolding or growing larger of a minute individual, preformed in the egg or sperm-cell, likewise it was assumed that a new generation lay in the ovary of that little being, with still smaller inviduals inclosed within their bodies, ad infinitum. This is called the box-within box theory. But exact study of the egg and the processes of embryological development show that such a miniature does not exist in the egg, and that development is not merely becoming larger. What is it that directs the course of de velopment and determines at what point new organs shall arise? We do not at present know. It is generally believed that the course of de velopment depends somehow upon the chemical constitution of nucleus and cytoplasm. As a piece of a hydra warps itself into an embryonic form and thereafter develops as the egg embryo develops, we must conclude that it is not so much the constitution of the egg or the sequence of eell-divisions that determines the form as some internal 'force' which acts throughout the entire organism. See EPIGENESIS; EtOLUTION (espe
cially remarks under Cooperative Evidences of Evolution); FCETUS; GEMMULE; GERM PLASM; ONTOGENY; PREFORMATION; RECAPITULATION THEORY; VESTIGIAL STRUCTURES, etc.
BIBLIOGRAPHY. Korsehelt and Heider, TextBibliography. Korsehelt and Heider, Text- Book of the Embryology of Invertebrates vol. i., translated by Afark and Woodworth; vets. iv., translated by Woodward (London and New York, 1895-97) : Balfour, A Treatise on Compara tive Embryology (London, 1881) : Hertwig, Text Book of the Embryology of Ilan and Mammals, translated by Mark (London. 1892) : Marshall, Vertebrate Embryology (London, 1893) ; Minot, Human Embryology (New York. 1892).