CHAPTER LIV 



ANIMAL DEVELOPMENT 

 PROPAGATION BY MEANS OF EGGS 



DEVELOPMENT FROM EGGS IN ANIMALCULES 



(PROTOZOA) 



The great majority of Animalcules are one-celled, a general 

 rule which is but little upset by the fact that a number of forms 

 are colonial, as the result of vegetative propagation. For in most 

 such cases there is no division of labour between the members 

 of the colony, all being precisely alike, and each of them having 

 to discharge the various vital functions on its own account. For 

 this reason a colony of this sort is said to be physiologically one- 

 celled. 



There can be no reasonable doubt that the remote ancestors 

 of Many-celled Animals were one-celled forms, resembling Ani- 

 malcules in essential respects, though many existing species of 

 these have become greatly specialized . on lines of their own, as 

 the result of division of labour within the unit masses of living 

 substance of which they are composed. And if this postulate 

 be admitted, the Metazoa may be imagined to have arisen from 

 colonial Protozoa in which the products of fission or budding 

 remained united together. The operation of the principle of 

 division of physiological work between the members of such a 

 colony would ultimately result in the evolution of different tissues, 

 i.e. groups of cells specialized in accordance with their particular 

 duties, and the colony would then be in effect a simple kind of 

 many-celled animal or Metazoon. Even the highest and most 

 complex Metazoa are after all cell-communities in which division 

 of labour is carried out into minute detail. An industrial illus- 

 tration may serve to make this clearer. Supposing we take all 

 the numerous processes involved in making boots to represent 

 the sum-total of vital functions. Then a boot-maker who carries 

 out all these processes with his own hands will correspond to a 

 Protozoon. A number of boot-makers working together in the 



