20 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



attained. In the animal bud morphogenesis usually occurs first at the 

 apex and progresses basipetally, and the region of most rapid growth may 

 also shift basipetally and perhaps becomes basal or posterior at some later 

 stage. In certain syllid annelids the posterior segment-forming region 

 buds, giving rise to new regions like itself, which become the apical regions 

 of the bud axes (except for the anal segment) but actually represent poste- 

 rior body regions and produce new segments anteriorly, the first formed 

 becoming the head (H. P. Johnson, 1901, 1902). In these buds the new 

 segment-forming regions remain embryonic for a time and give rise suc- 

 cessively to new segments; they and the posterior segment-forming regions 



Fig. 5. 



1933)- 



-Early stage in development of amphibian limb bud in section (from Filatow, 



of annelids in general, as persistent growing regions, are somewhat similar 

 to the vegetative tips of plant axes. 



In its most primitive form bud pattern becomes evident as primarily a 

 radial gradient system; but many buds, both plant and animal, show earli- 

 er or later a bilaterality or an asymmetry, usually definitely related to the 

 axes of the parent body. The bilaterality of many leaves and flowers, the 

 dorsivcntrality of lateral branches of many conifers and other plants, 

 and the anteroposterior and dorsiventral asymmetry of the amphibian 

 limb indicate that the axiate pattern of the region from which the bud 

 arises persists in, or is impressed on, the bud. 



In general, budlike developmental loci, whether plant or animal, wheth- 

 er developing into new individuals or into organ systems or organs, show 

 very similar gradient patterns in early stages. Unquestionably, the nature 



