STRUCTURE. 185 



are not wholly lost in the individuality of the aggregate, but 

 continue, some of them, to be displayed in various degrees: 

 the great majority of them losing their individualities more 

 and more as the type of the aggregate becomes higher. 



In a slightly organized Metazoon like the sponge, the sub- 

 ordination is but small. Only those members of the aggre- 

 gate which, flattened and united together, form the outer 

 layer and those which become metamorphosed into spicules, 

 have entirely lost their original activities. Of the rest nearly 

 all, lining the channels which permeate the mass, and driving 

 onwards the contained sea-water by the motions of their 

 whip-like appendages, substantially retain their separate 

 lives; and beyond these there exist in the gelatinous sub- 

 stance lying between the inner and outer layers, which is 

 regarded as homologous with a mesoderm, amoeba-form proto- 

 plasts which move about from place to place. 



Relations between the aggregate and the units which are 

 in this case permanent, are in other cases temporary: cha- 

 racterizing early stages of embryonic development. For 

 example, drawings of Echinoderm larvae at an early stage, 

 show us the potential independence of all the cells forming 

 the blastosphere ; for in the course of further development 

 some of these resume the primitive amceboid state, migrate 

 through the internal space, and presently unite to form 

 certain parts of the growing structures. But with the pro- 

 gress of organization independence of this kind diminishes. 



Converse facts are presented after development has been 

 completed; for with the commencement of reproduction we 

 everywhere see more or less resumption of individual life 

 among the units, or some of them. It is a trait of transi- 

 tional types between Protozoa and Metazoa to lead an aggre- 

 gate life as a plasmodium, and then for this to break up 

 into its members, which for a time lead individual lives as 

 generative agents ; and sundry low kinds of plants possessing 

 small amounts of structure, have generative elements — 

 zoospores and spermatozoids — which show us a return to 



