QUANTITATIVE ANALYSIS 117 



addition the same author (Ford 1931B) gives data on the 

 metamorphosis of the common eel (Anguilla vulgaris). Here 

 the tail shows positive heterogony during the process. An 

 analysis of Ford's table (p. 989) shows that when tail length 

 is plotted against rest-of-body length, its growth-coefficient 

 is at first over 1-4, then falls at first rather rapidly, then more 

 slowly to about i-i. It is, however, probable that it exhibits 

 a growth-gradient with centre of maximum growth anteriorly, 

 so that to get accurate figures for growth-coefficients we should 

 need data on a number of separate regions. 



It is clear that regions of different growth-coefficients will 

 often, though not necessarily always, be characterized by 

 special morphological differentiations. Each such region would 

 be characterized both by the possession of its own growth- 

 gradient and its own morphogenetic field (Guyenot). When 

 this is so, we may expect the region to be capable of extension 

 over more or fewer body-segments, and to cover a different 

 serial region of the body-segments in different individuals or 

 species. This would appear to be the case in regard to different 

 regions of the vertebral column (e.g. thoracic, sacral, cf. Bateson 

 1894). A good example of such a shift during individual 

 development is provided by the Copepods (cf. Caiman, 1909, 

 p. 89), in which a broad cephalothoracic region is sharply 

 marked off from a narrow abdominal region. The delimitation 

 of these two regions is not at a constant position in the body, 

 but moves back one segment at each moult, either for two 

 or for three successive moults. It would be interesting to 

 study the growth-relations of the segments which are thus 

 transferred from one region to the other : we may hazard 

 that they would be found to show a sudden alteration of 

 growth-intensity after their morphological changes, so as to 

 fit in with the growth-gradient of the anterior instead of the 

 posterior region. 



An interesting example of a general growth-gradient is 

 described by Faure-Fremiet (1930) for Zoothamnium alternans. 

 In this colonial ciliate there are two kinds of zooids, large and 

 small, the latter being in the majority. The multiplication 

 of the small zooids is not uniform ; but to use our author's 

 own words, ' decreases according to a kind of gradient in pro- 

 portion with its removal from the main strain.' It is this 

 growth-gradient which gives the colony its characteristic 

 form. The details are complex, and must be consulted in 

 the original paper ; but the existence of a gradient in power 



