SECT. 3] AND ORGANISATION 579 



found that a piece of the brain of a free-swimming larva would still 

 induce a medullary plate in the early embryo, so that the organising 

 power was still present, although it had long been unnecessary. 

 Again, Bautzmann transplanted a piece of notochord from a neurula 

 into the blastocoele cavity of a much younger embryo, and found 

 that the notochord fragment induced a medullary plate in the 

 ectoderm above it. It must, then, have retained the power to do so 

 long after it had been necessary to exercise it in the process of normal 

 development. Precisely analogous are the experiments of Wachs, 

 who found that regeneration of the adult amphibian lens takes place 

 under the inducing influence of the retina, just as had happened 

 originally in ontogenesis. Such experiments demonstrate that the 

 organiser persists into post-embryonic life. 



Spemann & Mangold also found that, when they transplanted 

 a piece of medullary plate into the blastocoele cavity of an un- 

 gastrulated embryo, it would induce in its turn another medullary 

 plate. This process, which they called " homoiogenetic induction", 

 is really a special case of the action of a second-grade organiser, in 

 which a tissue produces a replica of itself. 



A process occurring in the first and second periods of embryonic 

 development, which has not so far been touched on, is that of 

 "double assurance". Thus the eyeball in amphibia may induce a 

 lens in foreign epidermis, but in some amphibia the lens may develop 

 on its own in the absence of any eyeball, i.e. is self-differentiating, 

 and not dependent on the action of the optic vesicle in its capacity of 

 second-grade organiser. In Rana esculenta both faculties have been con- 

 clusively shown by Spemann and Filatov to coexist. Probably further 

 analysis of development will show that this double assurance principle 

 plays a great part in morphogenesis, and that cells only become what 

 they do under the influence of as many as three or four contributing 

 causes *. The double assurance principle may correspond to the 

 factors of safety which appear in structural engineering, so that, if 

 one process goes wrong, the embryo can still manage to complete 

 its development with the aid of the others. 



* Thus Bautzmann got differentiation of medullary plate from fragments of Triton 

 blastulae which lacked the entire organising region. Similarly Hoadley got self- 

 differentiation of parts of the chick blastoderm at 4-6 hours, whereas Waddington showed 

 such parts to be still plastic up to 18 hours or longer. It is clear that chemical 

 determination is often controlled by more than one agency, and that the times of activity 

 of these agencies may overlap. 



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