56 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tain oligochetes show such graded differences along the anteroposterior 

 axes of certain ohgochetes coinciding with the physiological gradients 

 present (Pickford, 1930; Sivickis, 1930). The functional activities of 

 various organs also give evidence of the presence of physiological gradients 

 of some sort. Transmission of impulses may occur more rapidly, or with 

 greater range, in one direction than in the reverse, as in the ctenophore 

 plate row (Child, 1917c, 1933a). Isolated pieces of the mammalian intes- 

 tine show a gradient decreasing posteriorly in rate of rhythmical contrac- 

 tion and a gradient in the same direction of excitability and various other 

 physiological characteristics (Alvarez, 1928). 



CONCLUSION 



It has already been noted that buds provide material much more 

 accessible than the ovarian oocyte for obtaining information concerning 

 the beginnings of developmental pattern. Development of an individual 

 from a bud is physiologically one of the most interesting and important 

 forms of development occurring under natural conditions. Bud pattern 

 may originate in a single cell or in a cell group which has previously con- 

 stituted a part of an individual, and cells from different parts of the indi- 

 vidual may give rise to buds. They have not had the same past history, 

 nor have they developed an organization, a pattern, like that of the egg. 

 As far as they are concerned, the new pattern may be entirely accidental: 

 they merely happen to be in the locus of the activation. The transforma- 

 tion of the radial gradient system primarily characteristic of buds into an 

 axial system by differential growth is highly suggestive for the problem of 

 axiate pattern. Is the physiological polarity which develops from the pri- 

 marily radial pattern anything more at its inception than an altered 

 spatial order of the radial gradient system? 



Fissions involve reconstitution with either modification of a pre-existing 

 partial pattern or development of new pattern; but they have the dis- 

 advantage, for certain experimental purposes, that they usually occur 

 only in certain regions of the parent body and under certain conditions, 

 though it is sometimes possible to induce them experimentally. The re- 

 constitutions following section can be initiated at will with variation of 

 form, size, region or origin of the isolated part, and physiological condition 

 of the parent individual, as affected by age or nutrition or as altered ex- 

 perimentally. Also, the effect of experimental conditions on developmen- 

 tal pattern of isolated pieces of different origin and in different physio- 

 logical condition can be investigated. Only certain directly distinguish- 



