182 PROBLEMS OF RELATIVE GROWTH 



growth appears to be of importance in holometabolous in- 

 sects in general, and in the dimorphism of male Forficula in 

 particular. (See Fig. 81.) 



Finally, anuran limbs well illustrate the dependence of 

 differentiation upon growth. It is often asserted that in 

 Amphibia the thyroid hormone ' favours differentiation but 

 inhibits growth '. This is inaccurate and misleading. Slight 

 thyroid activity, as in the normal tadpole, is not incompatible 

 with total weight-increase. Excess thyroid causes loss in total 

 weight, but this is an effect on balance, many organs losing 

 weight, others, like the limbs and skeleton, gaining weight. 

 As Champy (1925) has clearly shown, the effect of thyroid on 

 some tissues is to halt their growth (gut) or even to cause 



their atrophy (tail, gills), on others is neutral, 

 and on still others is to increase their 

 growth (limbs). Further, the differentia- 

 tion of the limbs is not a specific effect of 

 the thyroid hormone, but a secondary effect 

 of their growth in size. Differentiation of 

 limb-segments, digits, etc., occurs at certain 

 limb-sizes. And it will do so even when 



Fig. 81. Dispro- no thyroid hormone is present, as is shown 



portionately small by Allen's thyroidectomized tadpoles (Allen, 

 limbs caused by jqjg iqiq). These grew to a giant size 



precociously induced /r , 1 \ a • t 1 



metamorphosis in (for tadpoles). As result, their limbs, 

 the common frog, though growing isogonically, attained an 

 Jength S of°the "frogiet absolute size comparable to that reached 

 was 8 mm. by the limbs of normal tadpoles a few 



weeks before metamorphosis ; and they 

 showed a comparable degree of differentiation. This depen- 

 dence of type of differentiation upon absolute size of organ 

 is frequently to be met with (cf. in prawns with different 

 heterogony of the chelae in both sexes, the resemblance of 

 the proportions of heterogenic male and female chelae of the 

 same absolute size, but of very different ages and attached 

 to bodies of very different absolute size : see Chapter III). 1 

 In a later chapter we shall see that it has important taxonomic 

 and evolutionary consequences. 



The work of Hutt (1929) shows an interesting effect of the 

 male sex-hormone upon the proportion of limb bones in 



1 It is however not invariable. Male and female chelae of Maia, 

 Uca, etc., grow according to quite different growth-gradients (Chap- 

 ter III). 



