PREFACE xi 



Striking, and we may perhaps cite as particular examples such 

 works as Faure-Fremiet's Cinetique du Developpement ; Gray's 

 Experimental Cytology ; Dalcq's Bases Physiologiques de la Feconda- 

 tion ; and Needham's classic book on Chemical Embryology. 



So far, however, little progress has been made in equating the 

 results of the two lines of approach, and it seems clear that a con- 

 siderable time must elapse before it will be possible to do so satis- 

 factorily. At the moment the two fields are almost as unrelated as 

 were, through most of the nineteenth century, the cytological and 

 the experimental-genetic approaches to the problem of heredity, 

 which are now inseparable. 



That being so, we have not attempted to include the results of 

 the purely physiological study of development in this survey. This 

 means that we have deliberately excluded such topics as the 

 physiology of fertilisation, the mechanics of cleavage, and the bio- 

 chemistry of the egg and embryo, save where they have a specific 

 bearing on the biological problems involved. 



In other words, what we have attempted to do is to give some 

 account of the results of the experimental attack on the problem 

 of the biology of differentiation — the production of an organised 

 whole with differentiated parts out of an entirely or relatively un- 

 differentiated portion of living material. Almost the only short 

 books on this subject since Jenkinson's Experimental Embryology 

 and his (posthumous) Lectures are Brachet's L'CEuf et les Facteiirs 

 de VOntogenese, Diirken's Grundriss der Entwicklungsmechanik^ 

 Weiss' Entwicklungsphysiologie der Tiere, and de Beer's Introduction 

 to Experimental Embryology \ and each of these treats the subject 

 along rather different lines. Among larger works, Wilson's The 

 Celly Morgan's Experimental Embryology, Diirken's Lehrbuch der 

 Experimentalzoologie, and Schleip's Determination der Primitivent- 

 wicklung are the most important which have appeared since the 

 pioneer works on the subject. A perusal of them will suffice to 

 show the extreme diversity of their lines of approach. What we have 

 felt is that at present there exists in the subject a vast body of facts 

 and a relative paucity of general principles. We have accordingly 

 aimed at marshalling the facts under the banner of general prin- 

 ciples wherever possible, even when the principle seemed to be 

 only provisional. 



