The Determination of Size 



621 



(Berger, '37). The extent of correlation of 

 larval structure with stability of cell number 

 and organization deserves further study. At 

 the same time there is an implication, in 

 the glandular control of metamorphosis in 

 both insects and amphibians, of a shift or 

 increase in basal metabolic rate, and that 

 the collective changes amounting to meta- 

 morphosis may be the differential prolifera- 

 tive response of certain tissues to the new 

 metabolic level. 



In contrast to organisms whose size re- 

 flects the number of their constituent cells 



similar forms under varying conditions are 

 closely related. 



The main conclusion from Whitaker's 

 measurements is that, in general, eggs before 

 fertilization or activation are in a state of 

 abnormal metabolism compared with the 

 metabolism of adult tissues or of protozoa; 

 that this unusual condition is in some way 

 the basis of the attainment of large cell size; 

 and that with the restoration of normal me- 

 tabolism upon activation, cleavages auto- 

 matically follow until cell sizes associated 

 with normal metabolism are reached. Con- 



Table 28. Cell Diameters and Cell Numbers of Some Holoblastic Eggs 



(the number increasing with growth), are 

 certain small types that maintain a constancy 

 of cell number however much their size may 

 vary. Among these are the rotifers, phyl- 

 loxerans and Dinophilus (Van Cleave, '32). 

 Inasmuch as there is a limit to the size of 

 individual cells, it may be that these organ- 

 isms are limited to their peculiar small 

 dimensions simply by some inhibition of 

 cell division. 



The outstanding example of cells that 

 attain exceptionally large sizes are eggs of 

 virtually all kinds. Eggs of necessity are 

 cells large enough to yield through successive 

 divisions a number of smaller cells adequate 

 for the formation of some kind of active 

 organism. The problem presented by the 

 unusual extent of growth of oocytes is that 

 of limitation or determination of cell growth 

 in general, and here it may be most sus- 

 ceptible to attack. The most intensive investi- 

 gations to date have been those of Whitaker 

 ('33) on the changes in respiratory rates of 

 eggs of various marine invertebrates before 

 and after fertilization. Tyler's ('35) investi- 

 gations of respiration in fertilized eggs of 



sequently the curve of cleavage is similar 

 to the curve of growth, and represents the 

 progressive attainment of the new level of 

 equilibrivim or cellvilar steady state. 



The size of cells in differentiated tissues 

 varies considerably among different animals, 

 in amphibians relatively large, in Amphi- 

 oxus small, with the majority of forms be- 

 tween these two extremes. The cell number 

 at the end of embryonic development, that 

 is, at the end of the covirse of cleavage, is 

 accordingly a product of the initial volume 

 of the egg and the final average size of the 

 constituent cells. Table 28 illustrates this 

 for some holoblastic eggs (after Berrill, '35). 



The larval organism produced as the result 

 of cleavage and differentiation of the egg 

 is therefore more or less proportionate in 

 size to the mass of the egg, and in conformity 

 with this, whole larvae derived from half 

 or a quarter of the original egg, as from 

 isolated blastomeres, are proportionately 

 small. After cleavage as such has come to 

 an end and a functional larval organism 

 has been formed, growth proceeds anew as 

 food is obtained and utilized, a growth that 



