THE FUNCTIONAL PERIOD OF DEVELOPMENT 425 



The unification of the organism by means of the circulatory 

 system has in some ways a similar effect. It also makes possible a 

 competition between organs and regions for available nutriment, 

 and this may have marked effects upon development.^ The pro- 

 portions of parts of growing mammals, {a) fed maximally, {b) fed 

 so as to permit of only slight growth, and {c) fed so as to permit only 

 of maintenance of weight, are quite different.^ In extreme cases, 

 whole regions may disappear as a result of being drawn upon by 

 the rest. For instance, if a zooid together with an attached piece of 

 stolon of the Ascidian Perophora are isolated and starved in normal 

 conditions, the stolon will be completely resorbed by the zooid ; but 

 when placed in dilute toxic solutions the zooid is more affected, and 

 is then resorbed by the stolon (p. 294).^ In organisms without a 

 skeleton, starvation may produce reduction in total size, and then 

 different parts will be reduced at different rates, as for instance in 

 Planarians^ and in jelly-fish^ and hydroids.*^ 



The establishment of the circulation has a further consequence 

 which in vertebrates at least has far-reaching effects upon develop- 

 ment. It permits of the transport of hormones, some of which have 

 striking morphogenetic functions. Some hormones may be liber- 

 ated more or less continuously into the blood. This is apparently 

 the case with that amount of thyroid hormone needed to produce 

 normal development in man : when this threshold is not available, 

 the child is a cretin, stunted in growth and subnormal in intelligence. 

 In other cases, the hormones may be produced cyclically, and 

 this appears to apply to the hormone of the anterior pituitary con- 

 cerned with stimulating the cyclical growth of the ovarian follicles. 

 Or the hormones may be produced in markedly different amounts 

 as a result of nervous impulses to the gland, which in their turn are 

 controlled by external stimuli. In Amphibia, for instance, darkness 

 stimulates the post-pituitary to liberate the hormone which causes 

 expansion of melanophores : and while growth is taking place, this 



1 Roux, 1 88 1. 



- Jackson, 1925; Hammond, 1928; Huxley, 1932. 



3 Huxley, 1921 b. * Abeloos, 1928. 



^ de Beer and Huxley, 1924. ® Huxley and de Beer, 1923. 



