EMBRYONIC INDUCTORS AND ORGANIZERS 455 



through its consideration in handbooks, textbooks, general reviews, and 

 addresses.^ 



The induction which has aroused most interest and received most at- 

 tention, supposedly the earliest induction in amphibian development, is 

 the induction of the neural plate by the dorsal region which invaginates, 

 underhes the ectoderm which becomes neural plate, and itself becomes 

 chorda-mesoderm. Stages of invagination of chorda-mesoderm and ento- 

 derm and the neurula, with neural plate formed after invagination, are 

 shown in Figure 156. As is now well known from the work of many in- 

 vestigators, a piece of the presumptive chorda-mesoderm region (Figs. 149, 

 150), transplanted before its invagination to a region not normally in- 

 volved in neural-plate formation, or implanted in the blastocoel of another 

 embryo, can induce a new neural plate in ectoderm of the host which 

 would normally form only epidermis (Figs. 157, 158). The secondary em- 

 bryo thus induced was at first regarded as a product of the implant rather 

 than an induction (Spemann, 19 18), but heteroplastic transplantations 

 from the dorsal lip of the unpigmented Triton cristatus to the pigmented 

 T. taeniatus made it evident that the neural plate developing in relation 

 to the implant was wholly, or almost wholly, derived from host tissue, 

 while the underlying chorda-mesoderm was derived from the implant.^ 



Transplants of presumptive ectoderm had no such inducing effect but 

 were incorporated and developed according to their position in the host, 

 although preserving their pigmentary species-characteristics. The forms 

 resulting from these heteroplastic transplantations are chimeras, consist- 

 ing of tissue of two species. These experiments led Spemann to regard the 

 inducing region as an "organizer" or "organization center." Only some 

 of the more important points in the further investigation of various as- 

 pects of this induction by Spemann and many others are considered here. 



* See the following books: Morgan, 1927, a brief account; fuller consideration with ex- 

 tensive bibliographies in Schleip, 1929; Huxley and De Beer, 1934; Dalcq, 1935, on chordate 

 egg organization in general, with bibliography; Spemann, 1936, 1938, lectures concerned with 

 the problem of induction in embryonic development, with bibliography; Weiss, 1939, a general 

 textbook of experimental embryology; also the most recent book on the subject, Waddington, 

 1940, Organizers and Genes; a consideration, from the viewpoint of the Cambridge group, of 

 evocators and organizers in vertebrate, and chiefly in amphibian development, with discussions 

 of competence, individuation, and organization in general, and an attempt to bring genes 

 into the picture, but without anything new in the way of synthesis or further light on the 

 problems concerned. Numerous reviews of the subject have also appeared in Ergehnisse, 

 Jahresberichte, and review journals, notably those of Mangold, 1928a, 1929a, 1931a, with 

 bibliographies. See also De Beer, 1927; Gilchrist, 1929c; Weiss, 1935. 



'Spemann, 1921; Spemann und H. Mangold, 1924. See also Marx, 1925; Geinitz, 1925a. 



