The Determination of Size 



623 



The situation in birds appears to be much 

 more obscure. Byerly ('30) could find no 

 consistent weight differences between em- 

 bryos of the same age of White Leghorns 

 (adult weight, 2010 gm.) and Rhode Island 

 Reds (2770 gm.), although the smaller race 

 embryo was larger up to the tenth day, after 

 which the larger race embryos drew ahead. 

 Blunn and Gregory ('35), however, found 

 consistent but slight differences between the 

 two races (Table 30). 



Byerly, Helsel and Quinn ('38), studying 

 hybrids as well, found growth to be practi- 

 cally identical in rate between 2 and 20 days 

 for eggs of the same size, and egg size to be 

 more significant than genetic constitution. 

 Similarly, differences in gkitathione concen- 

 tration at the time of hatching appeared to be 

 too slight to be significant. After hatching, 

 the differential rate of growth becomes more 

 evident, and Gregory, Asmundson, Goss and 

 Landauer ('39) found correlated glutathione 

 differences. Kaufman ('30), comparing hen 

 and pigeon, could not find significant differ- 

 ences in growth rate during the embryonic 

 period, but considered the differences in size 

 of equivalent stages of the two to be due to 

 differences in the size of cells attained im- 

 mediately before cleavage; for example, liver 

 cells of the 3- and 11 -day embryos were 

 markedly larger in the chick. 



In contrast to increasing growth rates and 

 larger sizes associated with increase in gluta- 

 thione concentration are the problems of 

 growth inhibition. These are seen most 

 strikingly in the phenomenon of dwarf males 

 among various groups of animals, including 

 vertebrates. This may be regarded as a matter 

 of sex determination, the male condition 

 often being a consequence of small adult 

 size, particularly among forms normally 

 hermaphrodite. In the gephyrean Bonellia 

 indifferent larvae are produced, those settling 

 in mud growing to a large size as females, 

 those settling on the proboscis of a female 

 remaining minute and becoming male 

 (Herbst, '39). Nowinski ('34) and Baltzer 

 ('40) showed that the effective svibstance 

 for dwarfing was present in aqueous ex- 

 tracts and was heat-stable. Herbst, however, 

 found that various agents, such as low pH, 

 glycerol, traces of copper, and variations of 

 K* and Mg** also induced dwarfing leading 

 to male individuals. Dwarf males are also 

 found among cirripede crustaceans such as 

 Scalpellum, coexisting with large hermaph- 

 rodites. Reinhard ('42) is of the opinion 

 that in the Peltogaster, of the same group, 

 previously indifferent larvae settle on the ex- 



ternal sac of the parasitic hermaphrodite 

 and become dwarfed males in consequence. 



GROWTH RATE 



Growth rates are expressed either as math- 

 ematical formulae or as a growth curve, the 

 latter method being the more obviously in- 

 formative and more generally employed. In- 

 crease in length, area or mass, plotted against 

 time yields a typical growth curve (Fig. 

 214a). This is visually a svimmation or cu- 



Fig. 214. Curves of growth (from Medawar, '45). 

 (a) The curve of growth; (b) of growth rate; (c) of 

 acceleration; (d) of specific growth; (e) of specific 

 growth rate; (/) of specific acceleration. 



mulative curve and it expresses the successive 

 velocities or growth rates as a continuous 

 succession of varying magnitudes. On the 

 other hand, if the differences between the 

 values for successive units of time are plotted 

 against time, a velocity curve or curve of 

 the rate of growth is obtained (Fig. 2l4b). 

 One is the inverse of the other and each has 

 its value in making clear things which are 

 implicit but inconspicuous. 



Growth may be additive or multiplicative. 

 Additive growth is mainly confined to struc- 

 tures such as bone, teeth, scales, otoliths, 

 etc., and above all to the shells of moUusks 

 and Foraminifera. Multiplicative growth is 

 more characteristic of the growing organism 

 and differs in that the increment is itself 

 alive and growing, so that the growth rate is 

 not a fixed increment per unit of time but 



