712 PATTERNS AND PROBLEMS OF DEVELOPMENT 



of discovering whether, or to what extent, this capacity is present in par- 

 ticular primordia at particular stages. We know that in vertebrate de- 

 velopment different primordia attain the condition permitting independ- 

 ent differentiation at very different stages of development of the embryo — 

 some earUer, some later. Moreover, a part or organ system may be de- 

 pendent in certain respects, independent in others; an amphibian limb 

 bud, for example, is determined as a limb before its dorsiventrality is 

 fixed (pp. 285-88). The concept of mosaic development has in most, if 

 not in all, cases only relative significance. It was shown in chapter xiv 

 that even in organisms with determinate cleavage development is far 

 from being the complete mosaic that it was formerly supposed by some to 

 be. Even in amphibian development there is no single stage of develop- 

 ment of the whole organism at which it becomes a mosaic of independent 

 parts. Moreover, even though an optic primordium or a limb bud becomes 

 capable at a certain stage of more or less self -differentiation, the differen- 

 tiation of parts of these primordia is not independent of other parts and 

 in the urodele limb never becomes so. 



Capacity for independent differentiation does not necessarily involve 

 any visible differentiation; but in complex organ primordia, such as the 

 limb bud or the eye, a definite developmental pattern must be present 

 and must persist unaltered after isolation from the normal organismic en- 

 vironment. Considering, again, the amphibian limb primordium, it un- 

 doubtedly has become different in some way from other regions at a cer- 

 tain stage, whether we call this difference "chemodifferentiation," with 

 Huxley and De Beer, or "invisible differentiation" (Gilchrist, 1937) or 

 merely "determination." But for development as a limb, not only this 

 difference but an axiate pattern is necessary. Histological differentiation 

 of a single cell may result from chemodifferentiation of the cell, but for 

 orderly differentiation of an organ system independently of other parts a 

 developmental pattern is obviously necessary. In the limb primordium 

 this is apparently primarily a gradient pattern in the chemodifferentiated, 

 invisibly differentiated, or determined limb region. 



Independent differentiation of transplanted or isolated parts is further 

 illustrated by many other data discussed in earlier chapters. Amphibian 

 notochord, otic, branchial, and various other primordia acquire more or 

 less capacity for independent differentiation at a certain stage of their 

 development but at different stages of the embryo. Chorio-allantoic grafts 

 of portions of chick blastoderms show considerable differentiation in the 

 altered environment, but many of them give rise to more tissues or organs 



