CERTAIN GENERAL CHARACTERISTICS 21 



of the physiological differential from center to periphery differs in different 

 Kinds of buds, but the relation of the differential to center and to periph- 

 ery is very similar; and in the adventitious bud of the plant or the bud 

 which becomes a new zooid in the hydroid or a new appendage in the am- 

 phibian — the pattern — originally more or less radial, becomes longitudinal 

 in consequence of differential growth. 



Buds which develop into complete individuals must finally undergo 

 complete separation from the parent body. Usually the free end of the 

 bud axis becomes apical or anterior; the attached end, basal or posterior. 

 The fission which separates bud individual from parent occurs when the 

 attached region has attained a certain stage of development. A familiar 

 example is the hydra bud. In the buds of the segment-forming region of 

 certain syllids mentioned above, the head develops from the attached end 

 and its developmental stage determines separation, but the primary bud 

 pattern is apparently similar to that of other buds. So-called "buds" of 

 some forms — for example, the "winter buds" of some ascidian species — 

 are really fission pieces or cell aggregates rather than buds and separate 

 from the parent body before any development occurs. It seems preferable 

 to restrict the term "bud" to that form of development in which a new 

 axis originates in a local activation and differential growth. 



In some axes developing from buds or budlike outgrowths the region 

 of most active growth remains at the base instead of becoming apical. 

 The hairs of certain algae, the long bladelike leaves of various plants, and 

 probably some animal appendages are examples. These axes represent 

 progressive differentiation from base to tip during growth or development; 

 but to what extent this type of axis is an integrated whole, not merely a 

 differentiation gradient of a certain kind, is not known. This kind of bud 

 axis is much less common than that with the high end of the gradient at 

 the tip. Some of these "inverse" bud axes — perhaps all of them — are ap- 

 parently similar to the usual type of bud in their early stages, and in some 

 the region of most active cell formation and growth may shift from base to 

 tip in the course of development. Most of the bud axes mentioned in the 

 following pages are of the usual type, with the region of most intense ac- 

 tivation becoming the tip, at least primarily. The inverse bud axes usual- 

 ly, if not always, give rise to differentiated parts, not to entire individuals. 



Certain questions concerning bud pattern, conditions which determine 

 the origin of a bud, and other lines of evidence concerning its pattern, or- 

 ganism bud, and organ buds are taken up in later chapters. 



