PREFACE 



FOR the biologist there are, I conceive, in the main two 

 problems. One is to give an account of those activities or 

 functions by means of which an organism maintains its 

 specific form in an environment. The other is to find the 

 causes which determine the production of that form, whether 

 in the race or in the individual. The solution of the first 

 of these problems is the business of physiology, in the usual 

 sense of the term. The second falls to morphology. 



It is with the origin of form that we are here concerned, 

 and in particular with its origin in the individual. The 

 endeavour to discover by experiment the causes of this 

 process as distinct from the mere description of the process 

 itself is a comparatively new branch of biological science, 

 for Experimental Embryology, or, as some prefer to call it, 

 the Mechanics of Development (Entwicklungsmechanik), or 

 the Physiology of Development, really dates from Roux's 

 production of a half-embryo from a half-blastomere, and the 

 consequent formulation of the ' Mosaik-Theorie ' of self- 

 differentiation. That hypothesis has been the focus of much 

 fruitful criticism and controversy, the experiment has been 

 followed by many others of the same kind, and the present 

 volume is an attempt to sketch the progress of these researches 

 and speculations on the nature and essence of differentiation, 

 as well as of those which deal with growth, cell-division, and 

 the external conditions of development. 



In writing this review I have had the very great advantage 

 of an excellent model in the textbook of Korschelt and 

 Heider (Lehrluch der vergleichenden Entivicklungsgeschichte 

 der wirbelloten Thicre, Allgemeiner Theil, Jena, 1902). I have 



