206 THE FROG CHAP. 



one the ovum derived from the female, the other the 

 sperm from the male parent. Thus in passing from 

 the oosperm to the adult animal there is a gradual 

 structural or morphological differentiation accompanied by 

 a differentiation of function or division of physiological 

 labour. 



The expression " division of physiological labour " was 

 invented by the great French physiologist, Henri Milne- 

 Edwards, to express the fact that a sort of rough corre- 

 spondence exists between lowly and highly organised 

 animals and plants on the one hand, and lowly and 

 highly organised human societies on the other. In 

 primitive communities there is little or no division of 

 labour : -every man is his own butcher, baker, soldier, 

 doctor, etc., there is no distinction between " classes" 

 and " masses," and each individual is to a great extent 

 independent of all the rest. Whereas in complex civilised 

 communities society is differentiated into politicians, 

 soldiers, professional men, mechanics, labourers, and so 

 on, each class being to a great extent dependent on 

 every other. 



Development of other Organs Structure and Metamor- 

 phosis of the Tadpole Metamorphosis. A sketch of the 

 further development of the tadpole and of its metamor- 

 phosis has already been given (pp. 9-11, Fig. i), but 

 it is now necessary to add a few details to those already 

 mentioned. 



A third pair of branchial arches appear behind the two 

 already noticed, and on each a third external gill. The 

 first two pairs increase greatly in size and all the gills 

 become branched (Fig. 66, A), The four branchial 

 clefts communicate with the pharynx, and a current of 

 water enters the mouth and passes out by these clefts, 

 thus providing the gills with a constant supply of 



