286 PRINCIPLES OF EMBRYOLOGY 



progress in the study of growth as a precisely defined concept we shall have 

 to look to investigations on the synthesis of definable and isolatable pro- 

 teins, such as those of Spiegelman, Monod and others, on the rate of 

 formation of adaptive enzymes (pp. 400, 409), 



Meanwhile, empirical studies on the growth of the organism as a whole 

 present many points of interest which, however, there will not be space 

 to follow in any detail here. 



One most interesting and technologically important aspect of the matter 

 is in connection with the interaction between environmental and genetic 

 factors in the determination of growth rate and absolute size. Very little 

 indeed is known about the physiology of such process in animals, and they 

 present a promising field for investigation. Some work of potentially 

 fundamental importance is being made by the use of identical twin cattle. 

 The members of such pairs of twins have exactly the same hereditary 

 constitution. Bonnier, Hansson and Skjervold (1948) have shown that 

 their growth rates, although considerably influenced by the genes, can 

 be modified to quite an important extent by the level of feeding during 

 the growing period, but that two identical twins, one reared with 

 abundant nutrition and the other with much poorer supplies, will even- 

 tually tend to reach about the same final adult size although approaching 

 this at different rates. Again, King (1954) fmds that if a twin, is kept for 

 a period on a low-level diet and then changed to a high level, it soon 

 makes up for any stunting it may have undergone, and proceeds to grow 

 at the fast level characteristic of its abundant nutrition. After a certain 

 period on the high diet, it will be heavier than its co-twin if the latter 

 has been given the same diets in the reverse order, first high and then 

 low. 



The mechanisms controlling fmal size, i.e. the factors which cause an 

 animal eventually to stop growing, are scarcely understood at all. Some 

 species probably never cease growth; this is said to be the case for fish. 

 Others stop at a certain size, although the tissues are still capable of grow- 

 ing, and will do so if a part of the body is amputated. Others again (e.g. 

 mammals) grow till they reach a certain age, and, as we have just seen, 

 tend to reach a characteristic limiting size in the growing period. 



There may, perhaps, be no general mechanism which operates in all 

 these different situations; if there is, it is still obscure. Moment (1953) has 

 suggested that the limit might be set by the gradual building up of differ- 

 ences in electrical potential. Since tissues consist of cells, which are semi- 

 permeable bags containing electrolytes among which active chemical 

 changes are going on, it is to be expected that potential differences will 

 exist; and they have in fact been detected (Review: Lund 1947). Moment 



