112 



PROBLEMS OF RELATIVE GROWTH 



60 



Left Side 

 w 



20 



Right Side 

 W 60 



the gradient appears to be a continuous one. In other words, 

 although appendicular growth can only take place in relatively 

 small, pre-localized regions, yet the gradient determining the 

 amount of that growth is continuous. The growth-gradient 

 is thus probably something of a very fundamental nature, 

 akin to those gradients (of equally recondite nature) which 

 Boveri, Child, von Ubisch, Weiss and others have found it 



necessary to pos- 

 tulate in the early 

 development of the 

 egg and in regenera- 

 tion to account for 

 polarity and certain 

 orderly phenomena 

 of morphogenesis ; 

 the gradients hap- 

 pen to exist, and 

 where growth is pos- 

 sible, they influence 

 the amount of that 

 growth. We shall 

 be forced to draw 

 similar conclusions 

 in Chap. V (p. 152). 

 (And see p. 262.) 



However, that 

 such a general body- 

 gradient can be 

 locally modified is 

 clearly shown by 

 looking at the curve 

 for the right-hand 

 side of the males, 

 where a second high 

 point is made by 

 the chela. 

 Further proof of the graded regularity of the changes in 

 growth-intensity is seen when we investigate the asymmetry 

 of the hermit-crab in the two sexes (Bush's Fig. 9). Males and 

 females are approximately symmetrical in the head region 

 (though females appear to be slightly right-handed anteriorly, 

 slightly left-handed rather more posteriorly). The male be- 

 comes markedly right-handed in the chela region, and then 



Egestatks 



Antenna f 



Antenna 2 



Maxil/iped3 



Cheia 



PereiopodZ — 



Pereiopod3 

 Pereiopodt 



Pereiopod5 



Uropod 



60 W 20 W 60% 



Percentage increase in iengths of Appendages 



Fig. 61. — Growth profile of the hermit-crab. 



Growth-gradients along the body of male and 



female hermit-crabs (Eupagurus prideauxi). 



The figures give percentage increases of appendage length for a 

 given percentage increase of prothorax length (marked as dotted 

 line). 



