206 THE FROG CHAP. 



another both structurally and functionally of the adult 

 animal. So that every cell, fibre, or what not in the frog is 

 a linear descendant, through repeated fission, of the oosperm ; 

 and the oosperm itself is the product of the fusion of two 

 cells, 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 differen- 

 tiation 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 correspon- 

 dence 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, &c., 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. 



Structure and Metamorphosis of the Tadpole Develop- 

 ment of other Organs Metamorphosis. A sketch of the 

 further development of the tadpole and of its metamorphosis 

 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 branchial clefts 



