2 PATTERNS AND PROBLEMS OF DEVELOPMENT 



Workers in the ticld of developmental physiology have attempted by 

 many lines of experimental analysis to learn something more about devel- 

 opment than can be learned from observation alone. Their experiments 

 have not only given us a great body of data but have thrown much light 

 on various aspects of development concerning which we could have 

 learned little or nothing from mere observation, and have focused atten- 

 tion on various problems which promise to provide fruitful fields for fur- 

 ther investigation. 



To many biologists, even many students of developmental physiology, 

 the word "development" means embryonic development. They recog- 

 nize, of course, that other forms of development occur, but consider them 

 of secondary importance. As a matter of fact, embryonic development is 

 only one among many forms or types of development, and probably the 

 most highly specialized of all. Many organisms, both plant and animal, 

 develop in various ways which result either in similar or in different sorts 

 of individuals. For example, hydroids may develop from buds, and the re- 

 sulting individuals may be like the parent or different from it; they may 

 also develop from isolated pieces of the mature body, or in some cases from 

 aggregates of dissociated cells, as well as from eggs or parts of eggs, em- 

 bryos, or larval forms. Again, the organization of the ascidian egg is evi- 

 dently not essential for development of an ascidian, for an individual of the 

 same sort may develop from an egg, from the tip of a stolon which itself 

 originates as a bud, directly from a bud without stolon development, from 

 cell aggregates formed from parts of an otherwise degenerating parent 

 (the so-called "winter buds" of various forms), from experimentally iso- 

 lated pieces of the mature individual, either by direct reconstitution or 

 from cells of the piece remaining after partial degeneration, and from 

 pieces of stolon. Unquestionably, something about the physiology of de- 

 velopment of a hydroid or an ascidian is to be learned from all these differ- 

 ent ways in which it occurs, perhaps even more from some of them than 

 can be learned from the highly differentiated egg and its development 

 alone. Plants develop from eggs and from buds of various sorts originating 

 from sporophy te or gametophyte tissues, many of the simpler forms from 

 naturally or experimentally isolated pieces or single cells of the plant body. 



The occurrence of these many different forms or types of development 

 raises at once the question whether they are fundamentally different in 

 pattern or whether, in spite of the different starting-points, essential sim- 

 ilarity of pattern is to be found in some or all of them. Unless we are will- 

 ing to assume some metaphysical principle such as Driesch's entelechy 



