714 PATTERNS AND PROBLEMS OF DEVELOPMENT 



Annelid reconstitution shows essentially similar conditions. In all these 

 forms only the dominant region at the high end of the polar gradient is 

 capable of independent differentiation, and every body-level will develop 

 as dominant region unless it is subordinated to a dominant region or in- 

 hibited by external conditions (chaps, ix-xi). 



Within the hydranth primordium there is apicobasal dominance. Re- 

 moval of the apical region during hydranth reconstitution may result 

 either in regression of the remaining basal portion and reconstitution of a 

 new hydranth or in regeneration of the apical region, according to stage 

 and level of removal. In the planarian head the cephalic ganglia evidently 

 constitute the dominant region of the mature individual, and this domi- 

 nance is essential to persistence of the individual. The differentiation of 

 explanted planarian parenchyma into cells and fibers identical in appear- 

 ance with explanted ganglion cells and their outgrowths (Murray, 1927, 

 1 931) suggests that these ganglia are the self-differentiating regions of the 

 head in normal development. Even if this is true, however, the inde- 

 pendence is probably not absolute, for in grafts of the ganglionic region 

 into the ganglionic region of a host polarity of the graft may be altered 

 or reversed (p. 382). The mosaic characteristics of annelid and mollusk 

 development, as far as determined, concern larval parts which represent 

 early differentiations of anterior regions. There is considerable evidence 

 that the presumptive trunk region, at least in annehds, is not a mosaic 

 (p. 558). Even in these forms the difference between embryonic and adult 

 stages, as regards independent differentiation, is perhaps not as great as 

 has been believed. In short, in hydroids and planarians and apparently 

 in many annehds, subordinate parts are never emancipated; their per- 

 sistence as subordinate parts always remains dependent on dominance of 

 higher levels. Emancipation of parts or attainment of mosaic condition 

 is by no means a universal characteristic of development. Completely 

 dominant regions do not become emancipated but are independent from 

 their initiation. Other parts may, in some animals, attain capacity for 

 independent differentiation, either when they have undergone a certain 

 degree of determination or differentiation in relation to other parts or 

 when a local developmental pattern involving dominance and subordina- 

 tion has been established in them. Ability of a part to continue differen- 

 tiation for a time independently of its normal relations to other parts does 

 not necessarily prove that it is independent in the intact animal. Self- 

 differentiating parts may be accomplishing more in the way of develop- 

 ment than normally. Whether there is complete isolation or emancipa- 



