622 



The Determination of Size 



continues until sexual maturity is reached. 

 There is a change at the time a larva be- 

 comes active, from a condition comparable 

 to a closed system to an open one — from 

 tissues which are differentiating and deter- 

 minate to tissues that are functionally differ- 

 entiated and, for the most part, capable of 

 regeneration. The final limit to growth, and 



Table 29. Number of Blastomeres and Blas- 

 tocyst Diameter in Two Races of 

 Rabbits 



SMALL RACE 



LARGE RACE 



Average Number of Blastomeres 

 2>2}4 4.06 4.41 



40 8.29 9.94 



41 8.62 11.64 

 48 14.00 21.75 



144 



Blastocyst Diameter 



40.5/j 47.8m 



therefore size, which is reached during this 

 second phase of development, is genetically 

 inherent in the species and in general not 

 subject to experimental modification other 

 than a degree of stunting through some 

 form of malnutrition. There is, however, 

 some indication that growth of the body as 

 a whole proceeds until the gonads attain 

 their fimctionally differentiated state as 

 tissues. In many forms no further growth 

 occurs after this condition is reached. Even 

 where post-maturity growth is considerable, 

 there is a periodic spawning and a replace- 

 ment of gonadal tissue, so that the correla- 

 tion is still present. 



In meroblastic eggs the distinction between 

 embryonic development and post-embryonic 

 growth is less clear-cut. The embryo is 

 formed from the blastodisc and the scale of 

 its formation is there determined. The 

 greater part of the egg mass is drawn upon 

 by the developing tissues in a manner com- 

 parable with the nourishment of tissues in 

 the adult. The result is that the primary 

 embryo is a rapidly growing system and 

 not one merely undergoing cellular subdi- 

 vision and differentiation. In many such 

 cases there are three critical sizes in the 

 developmental cycle which are to some ex- 

 tent independent of one another: the size of 

 the embryo when it is more or less differ- 

 entiated, which is related to the initial scale 

 of organization; the size at hatching or, 

 alternatively, when the yolk has been ab- 

 sorbed; and the size at sexual maturity. In 



some instances of diapause in insects, for 

 example, in the grasshopper Melanoplus 

 (Burkholder, '34), the arrest of development 

 takes place approximately at the time of 

 full embryonic differentiation (growth to 

 the hatching size, when the yolk has been 

 fully incorporated), occurring only after dia- 

 pause has been broken. 



In meroblastic vertebrates, however, there 

 does not appear to be any such break in the 

 continuity of growth and the curve of growth 

 does not indicate the place where formation 

 of the embryo yields to growth of formed 

 structures, for growth is continuous through- 

 out the cycle, after the primary divisions of 

 the blastodisc are completed. This holds for 

 mammalian development as well as for other 

 amniotes, and hatching or birth in those 

 forms is little more than a temporary inter- 

 ruption of the growth process associated with 

 the change in the nutritive mechanism. 



One of the most significant leads in the 

 analysis of growth and size in amniotes was 

 given by Painter ('28) in connection with 

 the racial size differences between Flemish- 

 giant (5000 gm.) and small Polish (1700 

 gm.) rabbits. These differences were traced 

 back to early embryonic stages, even though 

 there are no differences in egg size. After 12 

 days' gestation the embryonic lengths were 

 23.1 and 18.1 mm., respectively, and he 

 concluded that the larger had correspond- 

 ingly more cells and therefore was growing 

 and proliferating at a greater rate, equivalent 

 organizational stages varying in size or scale. 

 These investigations were extended by Castle 

 and Gregory ('29), whose results are sum- 

 marized in Table 29. 



Table 30. Weights of Two Races of Chickens 

 at Three Stages of Development 



WHITE 

 LEGHORN 



RHODE ISLAND 

 RED 



Final size in these animals is accordingly 

 a reflection of early or even initial conditions. 

 It has been correlated with the concentration 

 of glutathione in the foetus at birth (Gregory 

 and Goss, '33; Goss and Gregory, '35; Ler- 

 ner, Gregory and Goss, '36), greater concen- 

 tration being associated with the higher 

 growth rate. 



