Ill 



INDUCTION GENERAL REMARKS 469 



clefts), feeding (stomodeum and proctodeum), hormonal integration (hypophysis, 

 adrenals, gonads, and probably other endocrine glands), motility (limbs), nervous 

 transmission and sensory perception, and reproduction, has undoubtedly played 

 a considerable role in evolution. Advantages capable of having a selective value 

 were attached to these improvements, which favored, refined, and amplified 

 definite functions of structures, and also increased the degree of freedom in develop- 

 ment (Medawar, 1954) and its opportunities for inventing efficient modifications. 

 Thus, processes of induction were a powerful instrument in the creative inter- 

 play between the new modes of development tentatively initiated by the various 

 chordates, especially vertebrate lineages, and the unavoidable struggle for life. 

 Vertebrates may be said to owe their existence to the invention and extensive 

 utilization of developmental inductions. Such considerations have some value for 

 other animal groups, which have also adopted mechanisms of induction, although 

 they are apparently different. For example, in insects induction acts mainly from 

 ectoderm on mesoderm and from mesoderm on endoderm (Bock, 1942; Haget, 

 1953; Krause, 1958); in echinoderms, induction probably helps the processes of 

 metamorphosis analyzed by Mc Bride (191 9) while for younger stages it is more 

 a possibility than a working mechanism; and in flat-worms, induction has been 

 demonstrated to be active in regeneration (Lender, 1952; WolfT, 1953)- 



(c) On the appreciation of the data 



Part of the fascination in the study of the essential aspects of life is due to their 

 elusiveness. In the last three decades investigations on induction have proceeded 

 in all imaginable ways without attaining a really decisive result. While the picture 

 has been considerably clarified, it has constantly become more complex, without 

 producing the evidence of one unequivocal solution. Confronted with such a 

 riddle, we have no choice but to estimate the relative value of the results and see 

 if they can be organized into a coherent interpretation. 



In this task, priority should undoubtedly be given to accurate observation, 

 mainly of normal embryos, enriched by comparison, and sometimes completed 

 by reference to suggestive experiments. This basic information will build up a 

 framework into which experimental data will have to be integrated. The classifi- 

 cation adopted in this review and essentially based on methodology, helps this 

 appreciation, but not less weight should be given to investigations described in the 

 last categories ; this particularly so if the conditions of the experiment, although 

 quite remote from the normal trend of ontogenesis, are more favorable for a 

 thorough analysis. 



(d) On the morphogenetic effects of induction 



Induction, as we have seen, performs several other tasks besides endowing the 

 embryo with a nervous system. In normal development, induction is responsible 

 for regional features of the uro-genital tract, the limbs, probably also the digestive 

 and respiratory tracts. Experimentally, the various parts of the organism may be 

 obtained with a relative selectivity, especially for the acromerit and the notomerit 

 or its hind parts. It is essential to note that their order of succession is rigid. As Rot- 

 mann (1949) noticed, it seems impossible to get either an acromerit followed by a 



Literature p. 483 



