82 PATTERNS AND PROBLEMS OF DEVELOPMENT 



ment potentialities may become progressively more limited, at least as 

 far as can be inferred from behavior of the material in certain environ- 

 ments; but we cannot be certain that in some other environment the po- 

 tentialities supposedly lost might not be realized. New potencies, how- 

 ever, may appear as development progresses: the ability to develop in a 

 certain manner is attained only when a certain stage is reached, that is, 

 when the developing system has attained a certain physiological condition 

 in consequence of changes occurring within itself, or in its intra- or extra- 

 organismic environment, or both. In case of a developing part of the intact 

 organism, both the part and its intraorganismic environment are appar- 

 ently factors in determining its developmental potency or potencies (cf. 

 Gilchrist, 1937a, ^). 



The methods most widely used in attempts to analyze potentialities and 

 potencies are those that are not infrequently regarded as the methods of 

 developmental physiology — isolation, explantation and transplantation or 

 grafting of parts of organisms, union of individuals, and isolation and ag- 

 gregation of cells. The method of differentially modifying development by 

 exposure of the whole developing organism or system to experimental en- 

 vironments, which was mentioned above under differential susceptibility, 

 also serves for the realization of other developmental potentialities than 

 those realized under natural conditions. 



ISOLATION OF PARTS 



Isolation of parts in experiment has usually meant physical isolation- 

 The part is separated from other parts and remains in the normal external 

 medium or may be brought into an experimental environment. An isola- 

 tion experiment involves at least two, often more than two, parts. A hy- 

 droid or a planarian may be separated into two or many parts; the blas- 

 tomeres of a two-cell or later stage may be isolated; a small part, an organ 

 primordium, a limb bud, the optic primordium, etc., may be isolated from 

 other parts, but those parts are also isolated from it, and experiment may 

 be concerned with the effect on either or both. 



Physiological isolation of parts without physical separation occurs in 

 many organisms in connection with agamic reproduction and with func- 

 tion and probably to a greater or less extent in development of the in- 

 dividual (see chap. ix). Results of physiological isolation are, in general, 

 essentially similar to those of physical isolation as regards further develop- 

 ment, though they may be less extreme. 



