14 PATTERNS AND PROBLEMS OF DEVELOPMENT 



ments there is complete degeneration of internal organs during an en- 

 cysted stage; nevertheless, development of planarians finally occurs. De- 

 velopment of the nemertean fragments is a reconstitution like that follow- 

 ing isolation of pieces by section. 



In development of new axiate individuals from buds there is much of 

 interest for developmental physiology. The bud makes its appearance as 

 a localized region of cell activation and growth, which in certain plants 

 may begin in a single cell of a leaf or other part of the vegetative body, 

 and in various unicellular organisms, in some portion of a cell. Buds of 

 many multicellular forms, both plant and animal, develop into new axiate 

 multicellular individuals or members of an individuation of higher order, 

 as in multiaxiate plants, hydroids, etc. Moreover, the individual which 

 develops from the bud may be like or unlike the individual from which the 

 bud arises or the individual which develops from the egg. Buds of some 

 forms from different regions of the parent body or from parents in different 

 physiological condition develop into different kinds of individuals; hy- 

 dranth buds and medusa buds of certain hydroids, and vegetative and 

 flower buds of plants, are examples. Many organs, such as tentacles, ap- 

 pendages, etc., originate as localized budlike regions of growth and differ- 

 entiation, apparently similar to buds which give rise to complete individ- 

 uals, but with limited developmental potentialities and often, like the am- 

 phibian limb bud (pp. 390-95), deriving a part of their axiate pattern 

 from the parent body. Fusion of eggs, embryos, or other parts may result 

 in the development of single individuals; and transplantation and im- 

 plantation of parts in embryonic and later stages make possible extensive 

 experimental analysis of determination in parts, of dominance, of induc- 

 tion, and of effects of different organismic environments, different gradi- 

 ent levels, etc., on the course of development. 



Since different levels of a physiological gradient differ in susceptibility 

 to many, if not all, external chemical and physical agents which are toxic 

 to living protoplasms, it is possible to alter and control the course of de- 

 velopment differentially by subjecting the whole organism to the action 

 of such agents. That the various forms of development provide practically 

 unlimited material for biochemical and biophysical investigation is ob- 

 vious. Needham's Chemical Embryology (1931), which is concerned with 

 the chemistry of only a single form of development, is sufficient evidence 

 on this point. And finally, it is evident that both genetic and environmen- 

 tal factors are concerned in all forms of development. All of them are to 

 be regarded as reactions of protoplasms of specific genetic constitutions to 



