XVIII SUMMAEY 659 



aided by the science of Experimental Embryology, founded by Eoux, 

 and so ardently followed by Drieseh, Herbst, and many others. If we 

 ask ourselves what is the most remarkable achievement of this science, 

 the answer must surely be the discovery of organ-forming substances. 

 We have already pointed out that such substances are emitted by the 

 nucleus of the ripening egg into the cytoplasm, and that they confer 

 on the latter a definite character ; and that the early course of develop- 

 ment consists in separating these substances from one another, and in 

 allocating them to different regions of the segmenting egg and embryo. 

 This separation can occur at an earlier period in one type of egg 

 (Annelida, MoUusca, Ctenophora, Tunicata), and at a much later period 

 in another type of egg (Hydrozoa, fechinodermata). Until it has been 

 effected a portion of the egg will produce a perfect embryo of 

 reduced size. 



We now see the inner meaning of that much debated " formation 

 of the layers," over which so many battles have been fought. Pressure 

 experiments prove abundantly that, after the primary emission of 

 organ-forming substances, the nuclei of the developing embryo form 

 an indifferent material, and what settles the fate of any cell is the 

 quahty of 'the organ-forming substance locked up in it. But, as we 

 have already pointed out, this indifference of the nuclei is a passing 

 phase, because, for one thing, if it were permanent, the male pro- 

 nucleus brought in by the spermatozoon would have no effect in 

 determining development, which is notoriously not the case. The 

 later course of development may be regarded as a result of the action 

 of already -formed organs on one another by the aid of hormones. 

 But indeed the organ-forming substances themselves are most plausibly 

 regarded as of the nature of hormones or ferments. This is clear from 

 the consideration that the quantity of the organ-forming Substance 

 can vary within wide hmits and yet give rise to a perfect organ. A 

 certain minimal quantity in the fragment of an egg wiU lead to 

 precisely the same result as a much larger quantity (cf. Boveri's 

 experiments on the eggs of Ascaris, Chap. XV.). 



We cannot for a moment imagine that the molecules of the organ- 

 forming substance are Kttle pictures of the organ which it is their 

 function to produce. Of course it is a mere commonplace to say that 

 we are only on the threshold of our knowledge of these substances, 

 and are still very far from understanding their modus operandi. 

 How is it, for instance, that a shght prick with a knife in the bud 

 from which a new tail is regenerated in a lizard which has lost its 

 tail, is capable of bringing it about that two tails, and not one, are 

 developed ? So far as our knowledge goes, it seems to be clear that 

 for the formation of an organ of what we may call the second order 

 (i.e. such a thing as a head or a limb), as distinguished from an organ 

 of the first order, or a germinal layer, the co-operation of at least two 

 organ-forming substances, in definite spatial relations to each other, is 

 necessary. 



If the spatial relations be altered we may get two organs formed 



