EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 295 



ment, first, from exactly similar cells, and more primitively from one 

 and a single cell the ovum or egg itself. Thus true is it that " all 

 the higher forms of life are aggregates of such morphological units or 

 cells, variously modified." 



But development teaches us something more. Every animal 

 above the rank of the amoeba and its kind and even these latter 

 may be included in the statement passes, in the course of its 

 personal progress towards maturity, through a stage in which the 

 original substance of the single primitive cell or egg breaks up into 

 numerous other cells, through the subsequent arrangement of which, 

 the body of the organism is in due course developed. This epoch, 

 our developmental studies have familiarised us with under the 

 designations of " segmentation " and the " morula-stage." In other 

 words, there is an early tendency on the part of every animal and 

 plant to depart from the single-celled stage, and to exhibit a com- 

 pound or collective structure. The egg, at first one cell, thus divides 

 to form a colony. Nor may the transcendental glance cease at this 

 stage of matters. If a colonial aggregation of cells at a very early 

 stage of development be a reality of life, if some animals, sponge 

 and hydra for example, are but collections of primitive cells, a no 

 less stable fact is expressed in the statement that in the adult body of 

 the highest animals such colonial aggregations are still to be traced. 

 Each tissue of the human frame, in its most vital phase, is a colony of 

 cells a compound cellular " individual," numbering its units by the 

 thousands, and possessing the power of growing, and reproducing new 

 cells, as truly as the zoophyte, by budding, repairs the constant loss 

 to which its component parts are subject. And there may further 

 exist in the highest animals, cells or units which exhibit well-nigh 

 as complete an independence of the frame in which they 'occur 

 as do the animalcular hosts outside. Thus the white corpuscles of 

 the blood (Fig. 198) of all animals exactly resemble amoebae in 



FIG. 198. WHITE CORPUSCLES OF THE BLOOD. 

 Different forms assumed successively by a white blood-corpuscle. 



structure, size, and movements. They are known to pass through 

 the walls of blood-vessels, to roam through the body at will, and 

 are seen to exhibit an utter and complete independence of all the 

 tissues of the body. More curious still, these white corpuscles have 

 been seen to ingest solid particles, exactly as an amoeba or allied 

 form receives its particles of food. It is not more wonderful, if 

 we think the matter over, to find in our own bodies many true 



