526 PATTERNS AND PROBLEMS OF DEVELOPMENT 



organ field is not necessarily limited to the region in which the organ ac- 

 tually develops but may take place in other parts of the field. Examples 

 of this in the limb field, the optic field, and some others and their apparent 

 relations to gradient patterns were considered in chapter xi. 



An extensive investigation of the capacity for development of isolated 

 parts of urodele and anuran gastrulae explanted in modified Ringer solu- 

 tion has shown that entoderm and parts of the mesoderm are capable of 

 advanced differentiation, even in small explants, but largely according to 

 prospective significance; that is, their development may involve more or 

 less reconstitution but is not greatly altered by isolation and explantation. 

 Parts of the dorsal inductor region similarly explanted show more exten- 

 sive reconstitution of parts beyond their prospective significance. Lateral 

 parts become bilateral; neural tubes and even epidermis may develop from 

 these pieces. Explanted ectodermal pieces show no capacity for any defi- 

 nite pattern of development. They form neural tissue only in relation to 

 an inductor. Also, gastrula ectoderm cultured for a time as explant and 

 then implanted in relation to an inductor shows decreasing reactivity, the 

 more advanced the stage from which it originates.'^ As regards the general 

 pattern of amphibian development, these experiments do not essentially 

 alter earlier conclusions. They confirm Holtfreter in his belief in a mosaic 

 of inductors in the dorsal inductor region, but this seems still to be a mat- 

 ter of opinion rather than a necessary conclusion from these or other exper- 

 iments. The data show great differences in capacity for independent de- 

 velopment ; but the question how far these differences depend on degree of 

 determination of parts, as Holtfreter believes, rather than on susceptibil- 

 ity to isolation and explantation is perhaps still open. It seems possible 

 that entodermal regions, for example, may be no more determined than 

 other parts in the sense of having attained a higher degree of specificity 

 before isolation but that, because of their low metabolism at the time of 

 isolation, they are not very susceptible to the altered conditions and con- 

 tinue development more or less according to the pattern of which they 

 were originally a part. Parts of the dorsal inductor region are much more 

 susceptible to isolation and show extensive reconstitution, even giving 

 rise to ectodermal parts. Explanted ectodermal pieces apparently never 

 attain a sufficiently high metabolic level to permit neural development. 



A very considerable capacity for reconstitution appears in various or- 

 gan primordia of later embryonic stages; for example, a complete limb 

 may develop from a part of a limb bud or prospective limb region. In this 



'5 Holtfreter, 1938^, b, c, and literature cited in these papers. 



