GROWTH 297 



jaws are usually modified in a way consistent with the shape of the upper 

 part of the skull to which they are attached. This subordination of indivi- 

 dual parts to the whole to which they belong is the general rule, but it is 

 not quite universal. For instance, in the Fg and later generations of certain 

 crosses between different breeds of dogs, some cases of definite disharmony 

 between upper and lower jaws and between other parts of the body may 

 be found, but they are relatively rare. Co-ordination usually extends 

 not only between parts of the same general nature, such as bones, but 

 affects tissues of quite a different kind, such as skin, muscle, etc. It is, 

 however, rather commoner to find instances in which the growth rates 

 of markedly different tissues are not properly assimilated to one another. 

 For instance, in dogs with greatly shortened faces, such as bulldogs, 

 the skin is often too large for the bony structure and therefore has to hang 

 in folds (Stockard 1941). 



Figure 13.8 



Skulls of various races of dogs, adjusted to the same size, to illustrate how 

 the structure, although complex, is modified as a whole, (a) German sheep 

 dog; (b) Borsoi; (c) German Dogge; (d) St. Bernard; (e) Zwergspitz; (j) 

 Itahan Windspiel; (g) Englischer Mops; (/i) Japanese spaniel. (After Huber 



1948.) 



Very little is known about the ways in which such growth correlations 

 arise. General endocrine control affecting all types of tissue certainly 

 plays a part in many cases. It seems probable also that the growth rate of 

 one organ or part of an animal usually has some influence on the growth 

 rate of the neighbouring regions. For instance, Huxley (1932) compared 

 the growth rate of the limbs in Crustacea in species in which one sex has 

 a very large, fast-growing hmb, which is absent in the other. He showed 

 that the presence of a fast-growing limb tended to affect its neighbours, in 



