628 



The Determination of Size 



voltage is thereby built up, thus stopping 

 proliferation at the growing end. Liebmann 

 ('45), however, attempts to correlate vari- 

 ability in oligochaete regeneration with the 

 state of the chloragogue system. 



The problem of size is emphasized even 

 more in the compensatory regeneration in 

 serpulid polychaetes and in certain decapod 

 crustaceans. In serpulids there is a pair of 

 opercula, one of which is large and serves 

 as a tube-stopper, the other being small and 

 rudimentary. If the large operculvun is re- 

 moved, the small one grows to replace it, the 

 stump of the large one regenerating merely 

 a new rudiment (Zeleny, '02, '05). Various 

 experiments show that the functional organ, 

 whichever one it is, holds its mate in check. 

 Okada ('33) found that the growth-inhibi- 

 tory influence is localized only in the oper- 

 culimi, is not due to a nervous reflex, and 

 is unlikely to be a diffusible chemical sub- 

 stance. 



The comparable reversal of large and 

 small claws in crustaceans has been discussed 

 by Zeleny ('05), Przibram ('31), Dawes 

 ('34) and Darby ('35). Dawes attempts to 

 apply the concept of allometry, claiming a 

 higher growth rate on the one side of the 

 animal as compared with the other, bringing 

 about an unequal distribution of substrates 

 for growth and causing one side to develop 

 a snap claw and the other a pinch claw. 

 Darby challenges this interpretation and sug- 

 gests that a secondary sexual hormone is 

 operating in conjunction with local selec- 

 tivity. 



In neither annelids nor crustaceans has 

 the matter been solved, and it remains the 

 triple problem of arrest of growth, alterna- 

 tive differentiation, and reversal of asym- 

 metry. 



Experimental investigations on the con- 

 trol of relative size have been made by 

 Harrison ('29), followed by Twitty and col- 

 laborators ('30-'40), employing transplanta- 

 tions of eyes and limbs of Amblystoma, 

 mainly between embryos and larvae of differ- 

 ent ages. When such organs are grafted into 

 hosts of the same species but different age 

 from the donor, regulation of size is accom- 

 plished by the organ growing slower or 

 faster than the corresponding organ of the 

 host, according to whether the host is the 

 younger or older. The growth rate of a 

 younger eye is accelerated, that of an older 

 one retarded, when grafted to a host of 

 intermediate age. 



Twitty suggests that size regulation during 

 development is determined as an equilibrium 



between the decreasing specific assimilative 

 capacity of the organ and increasing nu- 

 tritive conditions in the internal environ- 

 ment. Twitty and Wagtendonk ('40) found 

 evidence of a rise in nutritive level of the 

 blood with age, insofar as it depends upon 

 the amino-acid concentration. They interpret 

 the changes in the eye itself as a general 

 decline in the assimilative capacity of its 

 constitutuent cells. Whether growing cells, 

 and particularly those in progress of suc- 

 cessive divisions, undergo aging in this 

 sense seems doubtful. Pomeroy ('41) found 

 that in pigs the eyes and brain continue to 

 grow even under conditions of starvation, 

 so that what may be true for these structures 

 may not hold generally. It appears from 

 these investigations, taken in conjunction 

 with observations of Fankhauser ('45) on 

 the maintenance of normal structure in 

 heteroploid salamanders, that the size and 

 shape of organs and of the whole organism 

 are independent of cell size, and that cell 

 multiplication is subordinated to supra-cell- 

 ular controls. 



A parallel series of experiments involving 

 the transplanting of growing organs to hosts 

 respectively younger and older has been con- 

 ducted by Bodenstein ('40) with eye and 

 leg discs of Drosophila. His results are sim- 

 ilar to those of Twitty — discs transplanted 

 between larvae of the same physiological 

 age grew normally in their heterotropic po- 

 sition, while discs of the same age trans- 

 planted simultaneously into younger and 

 older hosts grew better in the older. In any 

 case the growth of the transplant is re- 

 tarded or accelerated until normal size rela- 

 tionship with the host organ is restored. 



A relationship similar to that of the sala- 

 mander eye and host exists between the 

 lens and eye vesicles of the same animal 

 (Harrison, '29). According to Ballard ('39) 

 the regulatory interaction concerns only the 

 retina and the cells, the mesodermal coats 

 of the eye and cornea having no specific 

 control of eye size. 



In general, nutritive level, oxygen tension, 

 etc., however much they may inflvience the 

 rate of growth and differentiation, have little 

 effect upon the size finally attained. Of 

 greater significance are factors that influence 

 the duration of growth without markedly 

 affecting the developmental rate. This is 

 suggested by experiments of Child ('39) on the 

 effect of time of development on Drosophila 

 wing size. At a constant temperature under 

 pseudo-starvation conditions involving the 

 use of ethyl parahydroxy benzoate, increased 



