626 



The Determination of Size 



not to affect their ratio. What is important 

 is to know how nearly constant k is, and to 

 what the constancy is due. The value of the 

 allometry equation depends upon this. 



allometric organ may shrink to relative in- 

 significance. If A; = 1, an organ exhibits 

 isometry and grows at the same rate as the 

 body. As a typical example, Huxley ('27) 



f 2 ^ 4 



-^^> 



^tT 



Fig. 216. Development of ascidian Botryllus buds from bud discs of varying size, with reference to 

 final size and extent of gonad development (from Berrill, '41). 



A, Discs of maximum sizes / — 4; B, closing spheres developing from discs / — 4, with extent of pre- 

 sumptive gonad area indicated in each case by black-out; C, continuation of same series 1 — 4, showing 

 failure of ova to segregate in / and 2, single and double segregation in 3 and 4; D, sphere stage / — 4 

 reduced to one-fifth previous scale; E, continuation of series 1-—4 on reduced scale as far as bud-disc, 

 gill-slit perforation stage, showing the development of the gonads in the four sizes; bd, bud disc; e, 

 endostyle; h, heart; o, ovum; t, testis. 



If k is greater than 1, the organ or di- 

 mension grows faster than the body or con- 

 trol dimension; if k is less than 1, it grows 

 more slowly. Given a suitable maximum 

 body-size a positively allometric organ 

 may grow relatively vast, while a negatively 



weighed the large chela and claw of 401 

 male fiddler crabs. He fovmd r = 0.0073:c^-^2 

 up to a total weight of 1.2 gm. (at which sex- 

 ual maturity is reached) and r = 0.083t^-255 

 thereafter. 



Lumer ('37) discusses the relation be- 



