ANTERO-POSTERIOR DEVELOPMENT 133 



of the difference. Unfortunately, Scammon's final extensive 

 data have only recently come into my hands, and time has 

 not yet been available for their full consideration and further 

 analysis in relation to the ideas set forth in this book. 



I incline to the view, after preliminary inspection, that 

 some of Scammon's data would be much better fitted by an 

 expression of the heterogonic type, y = ax k , than by the linear 

 formula he adopts. (It must, of course, be remembered that 

 his linear formula, when b = o, is a special case of the heter- 

 ogonic formula.) In any case, his analysis is valuable in show- 

 ing that the time-relations of development during the early 

 embryonic period of histo-differentiation, can exert a marked 

 effect on the percentage changes of parts in the later foetal 

 stage when auxano-differentiation is in progress. 



Latimer and Aikman (1931), in a study of the prenatal 

 growth of the cat from total weight 0-3 g. (264 specimens, 

 including 35 newborn), give further interesting data. 



The formula for the growth of the weights of various organs 

 (y) against total weight (x) are found to be as follows (all in 

 grams) : 



Head . y = x° ' 97 — 0-69^ 



Trunk . y = 0-59^ — 0-36 from 1 to 70 g. total wt. 



y = x 108 — 0-84* + 0-9 from 70 g. on. 



Fore-limbs (2) . y — o-o8# from 1 to 90 g. 



y = 0-053^ -j- 2-43 from 90 g. on. 



Hind-limbs . y = o-i* 11 — 0-053 from 1 to 100 g. 



y = 0-07,1; -f 3-5 from 100 g. on. 



Unfortunately in their tables the authors only give values 

 calculated on these empirical formulae, and the percentage 

 curves for relative weights in their figures do not appear 

 always to be consonant with the formulae. The percentage 

 curves are reproduced herewith. The constant decrease in 

 relative weight of the head, constant increase of that of the 

 trunk, and increase to a maximum followed by a decrease 

 for the limbs, is clearly brought out (Fig. 70). 



For an analogous case on invertebrates of the marked 

 effect which the law of antero-posterior development can exert 

 upon bodily proportions, reference may be made to the measure- 

 ments of Seymour Sewell (1929) on Copepods. During the 

 later free-living copepodid stages of development, before the 

 adult phase is reached, the number of abdominal segments is 

 increasing owing to the division of the less-differentiated sub- 

 terminal region of the abdomen. Growth is proceeding at 



