THE DEVELOPMENT OF ORGANISMS 743 



It is the transformation of the germinal organisation into the adult 

 organisation, and it implies a series of steps in differentiation and 

 integration. The fundamental fact which we are so far from under- 

 standing is that the fertilised ovum is at once the repository of ages 

 of organic inventions and a unified individuality in the one-celled 

 stage of its becoming. 



MODERN TRENDS IN EMBRYOLOGY— The old-fashioned 

 morphological embryography has largely given place to experi- 

 mental and physiological studies of development ; but the structural 

 (anatomical and histological) description of the stages in a life-history 

 can never cease to be an integral part of biology. Johann Schmidt's 

 masterly elucidation of the larval development of the Common 

 Eel has much interest even for the general biologist, and Leiper's 

 discovery of the story of the aberrant Trematode Bilharzia is notable 

 in itself, as well as in its practical applications. Of recent years there 

 has been no more striking achievement than Isabella Gordon's 

 description of the building up of the sea-urchin's test, from a few 

 spicules in the free-swimming larva to the elaborate edifice of the 

 adult. 



Notable advances have rewarded the applications of physiological 

 methods and ideas to embryology. There has been a fruitful study 

 of the regulative and "organising" influence of one part on another 

 during development, of the role of hormones in controlling the 

 rate and rhythm of developmental changes, of the significance of 

 certain environmental factors and chemical substances in the food. 

 The studies of Julian Huxley and others on de-differentiation and 

 regeneration illustrate the modem movement, and the experiments 

 of Spemann are outstanding. A few particular discoveries may be 

 mentioned, (a) In many cases, up to frogs, it is possible to induce a 

 quite normal development of eggs, although these have not been 

 fertilised, {b) Many experiments show that a part of an egg may be 

 as good as the whole. A larva may be reared from a fertilised frag- 

 ment of an Echinoid egg, or from one of the first four cells into which 

 the ovum of Amphioxus divides, (c) In certain cases, as in Cteno- 

 phores and Tunicates, it is possible to prove that there are specific 

 organ-forming substances or building-materials in an egg, whose 

 removal is followed by some particular defect in the developing 

 organism, {d) A portion of the optic vesicle of a tadpole grafted 

 under the skin of the larva in an entirely irrelevant place, such as 

 the side of the body, will induce in the cells of the epidermis the 

 formation of a lens ; and this evoking of lens-formation is the normal 

 function of the optic vesicle in its proper place. 



(e) If the newly fertilised eggs of the American Minnow (Fundu- 

 lus) are exposed for a few hours to a temperature a little above 

 freezing-point, a percentage will develop into blind larvae. This 



