NOTES ON EMBRYOLOGY AND CLASSIFICATION. 435 



lish the sharpest demarcation between such portions ; all 

 separation of constitutional and structural elements at one 

 time united comes under this head. 



Hypertrophy and Atrophy are the most obviously efficient 

 methods of differentiation when once a beginning has been 

 made, either by polar repetition or mere segregation. Hy- 

 pertrophy enlarges one unit or set of units, whilst the rest 

 remain stationary, or the one half of a cell already differen- 

 tiated by segregation is atrophied, whilst the other is hyper- 

 trophied. 



By Atrophy cilia disappear fspm the surface of the body, 

 and become confined to a single T)and ; by Hypertrophy this 

 band becomes drawn out into filamentous tentacles. 



The majority of developmental differentiations, which the 

 careful study of ontogeny and the cautious use of the " re- 

 capitulation hypothesis^^ enable us to infer as having histori- 

 cally occurred in the course of animal evolution, can be 

 reduced to terms of Hypertrophy and Atrophy. 



Concrescence finally, though a less striking, is by no means 

 a less important form of structural modification than those 

 which we have already noticed. Concrescence undoes the 

 work of Polar Repetition and Segregation. By it multi- 

 cellular tissues become syncytia, segmented animals lose all 

 trace of their segments, or their segmentation becomes 

 obscured and obliterated over large tracts. 



This is the process exhibited in the fusion of nerve-ganglia, 

 in the adhesion and combination of gill-filaments and in the 

 substitution of one continuous unit for a number. In 

 the last case it is not to be confused with hypertrophy and 

 concurrent atrophy. 



VII. Classification. 



Classifications of many kinds are possible and convenient 

 for various purposes in reference to the series of animal 

 forms. A classification may be said to be " subjective " or 

 to derive its importance from subjective relations which 

 sets up some particular characters chosen for reasons 

 best known to the person who chooses them, as the test- 

 points in reference to which animals shall be classified. It 

 has only lately been recognised as possible (namely since 

 the fact has been admitted, that all living and extinct 

 organisms are members of one great family tree) to investi- 

 gate the claims of a system of classification to acceptance by 

 reference to an objective standard. Whilst many classifica- 

 tions may be and are termed " objective" (because they are 

 logically correct), which, in consequence of their being 



vol. XVII. NEW SER. Y F 



