86 



PROBLEMS OF RELATIVE GROWTH 



joint. In almost all cases relative growth again falls off 

 steadily on the proximal side of the positive growth-centre. 1 

 The first and sometimes a few more of the basal segments 

 usually exhibit negative heterogony, but occasionally show a 

 very low positive heterogony. The first antenna as a whole 

 shows a slight negative heterogony relative to total length 

 (I.e., p. g). 2 



We may suggest that it is biologically desirable for the 

 terminal portion of the antenna to decrease, the proximal 



10 II 12 13 /■» IS 16 17 IB 19 20 21 22 23 24 2i 



antennal segments 



Fig. 47. — Change in proportions of segments of first antenna of Copepods 

 during growth. Constructed from the data of Seymour Sewell, 1929. He 

 gives the proportionate sizes of the antenna segments relative to total antenna 



length at various sizes. 



The graph gives the percentage change in proportionate size between the smallest and largest 

 stages measured (the segments have been grouped as indicated, and the means taken for the groups). 

 In every case the growth-rate of the basal region is low, usually negatively heterogonic ; there is a 

 centre of maximum growth at the 8th or 9th segment, and a centre of minimum growth at the distal 

 end. The transition from positive to negative heterogony occurs at about the 18th or 19th segment. 

 X Nannocalanus minor. + Eucalanus subcrassus. 



region to increase in relative size. Since changes in relative 

 size appear to operate by means of growth-gradients, the nega- 

 tive growth-centre in the tip of the antenna will be connected 



1 Seymour Sewell's data for Undulina vulgaris indicate that in this 

 species, after a low point of no change in proportions in the fourth 

 segment, the growth-gradient again turns upward as we pass towards 

 the body, which would give a still more complex growth-gradient. 



2 The increase in total body-length, however, is due partly to the 

 formation of new segments in the growing zone in the sub-terminal 

 region of the abdomen. As I shall attempt to show in a later chapter, 

 growth during early stages of the process, during which differentia- 

 tion from embryonic tissue is actively proceeding, obeys different laws 

 from those concerned with heterogony of parts which are already 

 differentiated. It would be better to compare the growth-rate of the 

 antenna with some definitely-formed part of the body, e.g. cephalo- 

 thorax-length, in which case it would probably show slight positive 

 heterogony. 



