PREFACE 



DEVELOPMENTAL physiology is often regarded as if it were 

 concerned only with embryonic development, and for some au- 

 thors of recent years it seems to be very largely a matter of verte- 

 brate or even of amphibian development. It cannot detract in any way 

 from the great interest and importance of the experimental work of Spe- 

 mann, his students, and many others on amphibian development to point 

 out that this and development of vertebrates generally constitute only a 

 small part and, so far as fundamental problems are concerned, probably 

 not the most important part of the field of developmental physiology. 

 Other patterns of embryonic development than those of amphibians or of 

 other vertebrates are of equal or greater significance in relation to the 

 physiological problems of development. Some of them appear to present 

 the problem of developmental pattern in simpler, more general form than 

 the vertebrates. 



Moreover, the animal egg at the beginning of embryonic development 

 gives little information concerning the beginnings and origins of develop- 

 mental pattern. Pattern is already present in eggs — often advanced in 

 development — when what we commonly call "development" begins, and 

 the ovarian developmental period is at present almost inaccessible to 

 analytic experiment. If we are to be consistent, we must admit that em- 

 bryonic development in general is by no means the only material of de- 

 velopmental physiology. There are many other forms of development, 

 with other starting-points than eggs. Some of them, such as buds, recon- 

 stitutions of isolated pieces, and development of cell aggregates, bring us 

 much nearer the beginnings of developmental patterns and the factors 

 concerned in their origins and permit more extensive control and analysis 

 than do most eggs and embryos. In fact, embryonic development appears 

 generally to be the most highly speciahzed form of development. The 

 animal egg is one of the most highly differentiated cells of the body. Only 

 by comparison and analysis of all forms of development can we hope to 

 distinguish the physiologically fundamental factors in developmental pat- 

 tern from those which are incidental to a particular kind of development 

 and to arrive at an adequate concept of the physiological foundations of 

 development. The following pages, though very far from accomplishing 



