Amphibians 



267 



duced cell mass could organize itself into 

 brain and sense organs. Very small masses 

 formed nondescript neural cell aggregates 

 or vesicles. Finally, there were intermediary 

 cell types between epidermis and neural 

 tissue known as "neuroid" or "palisade" 

 structures which were produced by the 

 weakest inductors (Holtfreter, '33c, '34a; 



DISTRIBUTION OF INDUCING AGENTS 

 IN THE ANIMAL KINGDOM 



It has been shown that inductions are 

 possible in combinations of tissues from dif- 

 ferent species or orders (p. 263). It was 

 known, furthermore, that the inducing ca- 

 pacity of at least some tissues is retained in 



Fig. 101. Fig. 102. 



Fig. 101. Induction of neural structures, obtained by placing a piece of prospective epidermis of the early 

 medullary plate into the blastocoele of an early gastrula of Triturus (from Holtfreter, '33e). 

 Fig. 102. Neural differentiations in fused pieces of prospective ectoderm of middle gastrulae of Aniblystoma 

 punctaturn, reared (without inductor) in salt solution (from Barth, '41). 



Needham, Waddington and Needham, '34). 

 It seems that the size of the induced area 

 plays a more important role in determining 

 the degree of complexity of the induced 

 structure than does any special property of 

 the inductor (see p. 278). 



The following conclusions may be drawn: 

 a chemical stimulus which is relatively stable 

 in extreme temperatures and in fat solvents, 

 rather than mere physical stimulation, is 

 instrumental in neural induction. Heating, 

 freezing and drying abolish the mesodermiz- 

 ing but not the neuralizing inductivity. 

 Furthermore, neuralizing agents are present 

 in all germ layers but are normally inactive 

 in the ectoderm and entoderm, either be- 

 cause they cannot diffuse out while these 

 tissues are alive (Holtfreter, '33e; Toivonen, 

 '40) or, more likely, because they occur there 

 in a physically masked, or chemically bound 

 form (Needham, Waddington and Needham, 

 '34) and are liberated only after the cells 

 are killed. 



stages in which the ectoderm is no longer 

 competent to react to them. For instance, 

 brain from a swimming larva (Mangold, 

 '29b) and somites and notochord from tail- 

 bud stages (Holtfreter, '33c, '36) proved to 

 be inductors. Woerdeman ('33b) reported 

 that even skeletal muscle and sarcoma from 

 adult rat, chick and man can neuralize gas- 

 trula ectoderm. Holtfreter ('34b) made an 

 extensive study of the inducing capacity of 

 a variety of tissues from larval and adult 

 organisms. Fragments from practically every 

 organ or tissue from various amphibians, 

 reptiles, birds and mammals, including man, 

 were inductive (Figs. 100, 103, 104). The re- 

 sults were about the same whether the grafts 

 were fresh, or had been dried, or briefly ex- 

 posed to higher temperatures. Likewise ac- 

 tive were cell-free coagulated homogenates 

 from amphibian and chick embryos and from 

 Daphnia and beef liver. Most tissues tended 

 to induce neural structures with cephalic 

 characteristics, while others (e.g., liver, kid- 



