STRUCTURE. 185 



arc 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 mesodcrm, amceba-form proto 

 plasts which move about from place to place. 



Eelations 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 Eehinoderm larva; 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 amoeboid 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 ; anil sundry low kinds of plants possessing 

 small amounts of structure, have generative elements 

 zoosporcs and spermatozoids which show us a return to 



