132 PROBLEMS OF RELATIVE GROWTH 



§ 6. The Law of Antero-posterior Development and 

 its Effect upon Growth 



In this connexion, we have the important work of Scammon 

 (references in Scammon and Calkins, 1929) on the growth of 

 the human foetus, from about 5 cm. length to birth. He there 

 finds definite evidence of gradients as regards growth, proceed- 

 ing antero-posteriorly along the main axis, and centripetally 

 along the limb axes. This main gradient is found for internal 

 organ-systems (gut, vertebral column, etc.) as well as for 

 external form. There are a few exceptions (e.g. the sacral 

 region of the vertebral column) which apparently, like in- 

 tercalated centres of high growth-ratio in Crustacea, such 

 as the large chela of male Uca, are concerned with special 

 adaptive growth of particular organs. 



However, he asserts that practically all parts so far measured 

 grow in linear proportion to crown-heel length, according to 

 the simple formula y = ax + b. He further points out that 

 when b is zero, the percentage measurements of the organ, 

 relative to standard length, remain constant throughout the 

 period. If, however, b is positive, the percentage measure- 

 ments decrease with increase of absolute size, while the converse 

 is the case if b is negative. Since the anterior regions have b 

 positive, while in the posterior regions it is negative, there 

 is the appearance of a growth-gradient. However, this would 

 not be brought about, as in my previous examples, by differences 

 in growth-intensity of the various parts, as measured by 

 constant differential growth-coefficients, but according to 

 whether the organ in question had made much or little growth 

 during the embryonic period (below 5 cm. length). This he 

 would interpret as due to the Law of Developmental Direction, 

 according to which anterior (and proximal) regions are formed 

 first, are soonest through with their origin and histological 

 differentiation, and can embark earlier on their main growth- 

 period. As we may presume that the growth of each separate 

 organ follows the usual rule for the body as a whole, namely 

 that the (compound interest) growth-rate slows off progres- 

 sively from the beginning of growth, we should expect that 

 there would be a lag between anterior and posterior regions, 

 such as that at any given moment the anterior ones would be 

 at a later and therefore slower phase of their growth than 

 those lying more posteriorly (and see §7). 



It remains to be seen whether this will account for the whole 



