214 GROWTH PRINCIPLES AND THEORY 2 



rate, e.g. in mice, is less in animals born in small litters and therefore having higher 

 weight at birth, than in animals born in larger litters and with a smaller birth 

 weight (Kopec, 1932). 



The principle of equifinality is also applicable to the balance of organs and of 

 chemical constituents in the animal organism (p. 229ff.). Apparently a certain ratio 

 between the size of the individual organs and the size of the body must prevail if 

 the organism is to function properly. However, different species are born in 

 widely different stages of development. In order eventually to reach the functional 

 balance of organs and constituents, relative growth of organs has to be different 

 in different species. This implies that the same final result, that is, a balance of 

 organs as expressed in interspecific allometry, will be reached in different ways, 

 that is, by allometric growth differing intraspecifically [cf p. 241 ff.; Bertalanffy 

 and Pirozynski, 1952). 



Equifinalities which cannot be quantized but apparently are based upon similar principles, 

 are found in many processes of development. For example, the stages of the gastrula, that 

 is, a two-layered embryonic stage with ectoderm and endoderm, and of the neurula 

 with its characteristic arrangement of archenteron, notochord, neural tube, and mesoderm, 

 are found universally and appear to be fixed points which may be reached in quite 

 different ways. The process of segmentation is different in the several animal classes and 

 orders; owing to the yolk content of the ovum, equal, unequal, discoidal, etc., segmentation 

 may take place. A coeloblastula or a sterroblastula may result. Gastrulation may occur 

 by the ways of invagination, epiboly, immigration or delamination of cells, or a combi- 

 nation of these processes; but nevertheless a similar two-layered gastrula stage is eventually 

 established. This can be seen, for example, in the various types of gastrulation found in 

 scyphozoa or in polychetes; or experimentally, if the normal unequal segmentation of 

 amphibian eggs is changed into discoidal segmentation by centrifugation, without this 

 having an influence upon subsequent development. Similarly, early development of ova 

 o( Ainphioxits., selachians, amphibia, and reptiles is widely different; but the neurula even- 

 tually developing in these classes is surprisingly similar. Again, the development of the 

 vertebrate liver takes place in very different ways, and no less than 12 different ways of 

 development were found in 30 species investigated (Elias, 1955). In striking contrast, 

 no vertebrate organ shows more uniformity of structure in the adult stage than the liver. 

 As justly stated by Elias, this is in contradiction both to \'on Baer's law according to which 

 embryos of different species are similar and diverge later on in the adult stage; and to 

 Haeckel's biogenetic law since the embryonic liver does not recapitulate the structure 

 of ancestral forms in phylogeny. 



Phenomena of regulation, considered by Driesch (1929) as a "proof of vitalism", appear 

 to fall into the same category. The same final stage, a normal and complete larva, can 

 develop from a complete normal ovum of the sea urchin, from each half of an ovum divided 

 in two, from two ova fused into one, etc. The same applies to regulation in ova in general 

 and to early organ anlagen such as those of the limb, eye, and heart. According to Driesch, 

 this equifinality of development can only be explained by the intervention of a vitalistic 

 entelechy which directs the process in order to establish a complete organism. However, 

 attaining the same final state from different initial conditions is by no means a vitalistic 

 feature exhibited in processes of this sort; rather it is a general characteristic of open 

 systems. 



Similar considerations apply to regeneration. Here we find the paradox that seemingly 

 lack of a part which was lost acts as a releasing stimulus for restitution of that part (A. E. 

 Needham, 1952). How can something that is non-existent be a causative agent? However, 

 the paradox disappears and there is no need to hypostatize hypothetical regeneration- 

 inhibiting substances in the intact organism if it is realized that the organism, by regene- 

 ration, tends to reestablish an equifinal steady state. We are far from being able to give 



