EMBRYONIC INDUCTORS AND ORGANIZERS 473 



dition, regions of origin, and developmental stage of ectoderm or other 

 tissue in which induction occurs — play an essential part in determining the 

 result, particularly when pieces of tissue are explanted with the inducing 

 tissue. The effects of foreign inductors are found to be quantitatively 

 greater at anterior than at posterior levels of the hosts. Organs of the 

 head region, brain, sense organs, and balancers appear chiefly in tissue 

 from more anterior levels, while posteriorly atypical neural structures and 

 tails are more frequent; induced kidneys and legs appear more frequently 

 at or near the levels where they normally develop. Also, axial orientation 

 of induced parts, if they are axiate, commonly coincides in direction with 

 longitudinal host axis (Holtfreter, 1934/^). The anteroposterior differences 

 and the orientation point to the longitudinal or polar gradient of the host 

 as the factor concerned, and the regional differences in character of organs 

 induced may also be determined by this gradient or by the regional or or- 

 gan fields resulting from it. If this is the case, an adequate activation of a 

 region of the host body within a particular field may be expected to result 

 in development of the organ or organ system characteristic of that field. 

 However, correspondence between expectation and actual character of 

 induced parts is by no means complete. Apparently the fields, if present, 

 are not stably determined, and activation may have different results in 

 the same region. Brain and head structures are said to appear more fre- 

 quently with strong than with weak inductors. If the inductor is primarily 

 an activator, this is to be expected. A weak inductor may activate only 

 slightly and so induce only some slight differentiation of neural tissue or 

 only a thickening that cannot be certainly identified as neural, even in 

 anterior host regions. Conversely, a strong inductor may activate more 

 posterior levels to such a degree that they develop as brain. Still more in- 

 tense activation may perhaps induce formation of notochord and meso- 

 derm. It is difiicult to account for the experimental results except in some 

 such terms as these, unless we assume specifically different inductor sub- 

 stances for each of the different inductions. 



A recent example of such assumptions appears in the work of Chuang 

 (1938, 1940; also further data in another paper, 1940, Arch. Entw'mech., 

 140). He finds that mouse kidney induces in isolated gastrula ecto- 

 derm exclusively brain parts, sense organs, and other head structures 

 and that Triton liver induces trunk regions, notochord, muscles, and tail. 

 These differences are regarded as specific effects of the two foreign in- 

 ductors. With implantation of the kidney or liver in the ventral side of a 

 gastrula these differences in character of inductions are less evident; both 



