CHAPTEE II 



THE MECHANISM OF HEREDITY 



A. — Reproduction in the Protozoa and Simple Metazoa. 



The world of organic nature is divided into two great groups— 

 the unicellular organisms (Protozoa and Protophyta) and the 

 multicellular organisms (Metazoa and Metaphyta). 



The Protozoa, or unicellular animals, differ from the multi- 

 cellular animals not merely in the number of their component 

 cells, but also in the relative absence of that differentiation 

 which in the Metazoa is associated with the ever-increasing 

 division of labour. Whereas the Protozoa are composed of one 

 single cell, which serves for locomotion, digestion, excretion, 

 reproduction, and all the other functions of this elementary 

 life ; the Metazoa, with the exception of their lowest forms, 

 possess at least one general differentiation — that between repro- 

 ductive and somatic cells. Among the lower forms of Metazoa 

 a differentiation of the cells into reproductive and somatic is 

 still incipient ; but in most cases this fundamental differentiation 

 is distinct between the cells of the soma, adapted to the mainte- 

 nance and repair of individual Ufe, and the reproductive cells, 

 adapted to the continuance of the life of the species. 



Among the simplest multicellulars we find some intermediate 

 forms — often reckoned as Algae— between the Protozoa and the 

 higher forms of life. These simple multicellulars differ from 

 the uniceUulars only in the fact that they are composed of a 

 large number of cells, for these cells are as yet wholly undiffer- 



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