EMBRYONIC INDUCTORS AND ORGANIZERS 471 



the neural plate remain capable of induction after killing by heat, freez- 

 ing, drying, after several hours in 20 per cent HCl, several days in ether, 

 several hours' extraction by ethyl or petrol ether, 6 months in alcohol, fol- 

 lowed by impregnation with xylol and paraffin, etc."' This line of experi- 

 ment was most extensively developed by Holtfreter, using the method of 

 implantation in the blastocoel of a host and that of culture in a modified 

 Ringer solution of pieces of presumptive epidermis placed on, or inclosing, 

 the treated tissue. Equally interesting was the discovery by Bautzmann, 

 and its confirmation and extension by Holtfreter, that regions of the em- 

 bryo which do not induce when alive can become inductors when killed in 

 various ways. Even fragments of boiled undivided eggs and centrifugates 

 of ovarian eggs are able to induce. It was also found that various adult 

 tissues from many animals and even some plant tissues have more or less 

 inducing power on urodele embryos, some both hving and dead, others 

 only when dead.'" Apparently, however, hquid tissue extracts, to be effec- 

 tive, must be coagulated or more or less solidified by addition of sub- 

 stances proved not to be inductors, but solid bodies are not necessarily 

 inductors. The heterogeneous character of these foreign inductors is evi- 

 dent from the following incomplete list, taken in large part from Weiss 



(1935)- 



Plants: Cambium of birch, induced small neural plate; growing tip of potato tuber, 

 induction neural of tissue but no distinct plate 



Coelentera: Hydra tissue, boiled 



Annelids {Enchytmeus) : Body fragments 



Mollusca {PlanorUs, Limnaea) : Muscles of foot, hepatopancreas 



Crustacea {Daphnia) : Coagulated body extract. 



Lepidoptera {Deilephila) : Haemolymph and ganglia of pupa 



Odonata {Libellula larva) : Fat body, ganglia 



Fishes: Gasterosteus: heart, liver, ovarian eggs, muscle, spleen; Danio: presump- 

 tive chorda-mesoderm 



Amphibia {Triton, Salamandra, Rana): Liver, heart, ovarian eggs, nuclei and 

 cytoplasm of unfertilized eggs, nuclei more effective, muscle, cartilage, brain, retina, 

 regenerating tissue of tail 



Reptiles (Lacerta) : Liver, kidney, testis 



Birds: Liver, kidney, testis, thyroid, fat body, brain, retina, coagulated chick 

 embryo extract, fragments of primitive streak 



Mammals (mouse) : Heart, liver, kidney, adrenals, brain lens; also calf liver 



Man: Liver, brain, kidney, thyroid, tongue, sarcoma, carcinoma 



2' Bautzmann, Holtfreter, Spemann, and Mangold, 1932; Mangold, 1932b; Holtfreter, 

 1933c, 1934a; Spemann, Fischer, and Wehmeier, 1933; Wehmeier, 1934. 



" Fischer und Wehmeier, 1933a, b; Woerdeman, 1933c; Holtfreter, 1934^/ Wehmeier, 1934; 

 Hatt, 1934; Waddington and Wolsky, 1936; Ragosina, 1936, 1937; and other papers. 



