ANTERO-POSTERIOR DEVELOPMENT 



137 



total length, m. 



17 20 23 





To obtain accuracy I have recalculated the absolute data 

 for head-length on the assumption that the mean total length 

 for Class 1-2 m. is 1-5 m., and so on ; the figures appear in 

 Column 4 of the table. A log-log graph from these (for post- 

 natal life) is given in Fig. 71. Up to 25 m. total length, it 

 shows a remarkable approximation to a straight line, giving 

 a growth-partition coefficient of head-length relative to total 

 length of about 1-55. When the figures for foetal life are 

 plotted, they also show a 

 straight line, but indicating 

 a growth-coefficient of only 

 about 1-05. The two lines 

 intersect at about 17 m. 

 total length ; if this gives a 

 correct indication, it means 

 that the marked heterogony 

 of the head does not begin 

 until long after birth, which 

 appears to occur at 6-7 m. 

 length, but before sexual "« 

 maturity, which comes on 

 at 22-23 m - length. 



Numerous further excep- 

 tions can be found among 

 the limbs of Arthropods. 

 As a simple example, we 

 find that in primitive Cope- 

 pods both the first appear- 

 ance and the subsequent 

 growth of the appendages 



occur in strict antero-posterior sequence ; but in many of the 

 more specialized members of the group, while the appear- 

 ance of the rudiments still takes place in this sequence, 

 the swimming feet of the anterior thoracic segments then 

 grow rapidly, while the more anterior maxillae and maxil- 

 lipeds remain for some time in a rudimentary bud-like con- 

 dition (see Caiman, 1909, p. 89). Scammon himself mentions 

 some exceptions to the purely linear growth of parts. The work 

 of Schultz (1926, 1930) 1 on other Primates refers to various 



1 Schultz presents his results almost entirely in the form of percentage 

 values (ratios) and their changes with age. It would be of considerable 

 interest to undertake an analysis of his absolute data to see whether 

 they conformed toScammon's linear or to my heterogonic formula. 



Fig. 71 . — Head length against total length 

 in male Blue-whales ; logarithmic plot- 

 ting, k = 1-55 (except for last point). 

 (From Data of Mackintosh and Wheeler, 1929.) 



