SECT. 3] AND ORGANISATION 581 



surface is directly proportional to the amount of vegetable matter 

 in the diet; if it is large, the intestines are capacious and long, if it 

 is small, the intestines are small. In this connection the case of the 

 changes in the intestinal tract of the opossum studied by Heuser 

 is of interest, for it has a gestation time of only 1 3 days, and has to 

 live some time on milk in the pouch. Babak's work was criticised by 

 Klatt, but has been confirmed more fully by Elven. 



The three periods in the life of the embryo are now known to 

 have different relations to regeneration. Przibram's "law of apo- 

 genesis" holds true in the main, namely, that the younger an animal 

 the greater are its powers of regeneration (cf. the work of Abeloos 

 on Planaria dorotocephala) . But this must be qualified by the statement 

 that, during the intermediate period, no regeneration is possible. 

 Each individual part and organ of the embryo has its work cut out 

 to differentiate into its destined form, and any replacement of lost 

 parts cannot be made. Thus Spurling found that the limb buds of 

 the chick cannot be reformed during this middle period. But in the 

 later period of functional differentiation, regeneration is possible, 

 as has been shown by Olmsted; Morgan & Davis; Davidov; and 

 Nussbaum & Oxner. Perhaps the best study of this is the work of 

 Mackay, Mackay & Addis on compensatory hypertrophy after uni- 

 lateral nephrectomy. Their figures were as follows : 



Age in months % hypertrophy 



I 52-6 



3 36-7 



6 32-8 



12 32-2 



It is in this later period of functional differentiation that Roux's 

 doctrine of the struggle of the parts has its significance. The signs 

 of this equilibrium between organs and tissues do not emerge except 

 when the organism is subjected to the stress of an unfavourable 

 environment. Then some parts will be found to have the preference 

 and to take a relatively greater share than the others in the available 

 food-supply. The reproductive glands have an important position 

 here, e.g. the testis of the starved rat in Siperstein's work, and there 

 are many instances where the ova in course of preparation draw to 

 a great extent upon the remainder of the body. A discussion of these 

 facts will be found in the appendix on the maturation of eggs ; here 

 it may suffice to mention the work of Greene on the salmon, who found 

 a remarkable constancy in the chemical composition of the ovaries, 



