MOErHOLOGICAL SUMMARY. 109 



Thus the body of the cra^yfish is resolvable, in the first 

 place, into a repetition of similar segments, the metameres, 

 each of which consists of a somite and two appendages ; 

 Hie metameres are built up out of a few simple tissues; 

 and, finally, the tissues are either aggregates of more or 

 less modified nucleated cells, or are products of such cells. 

 Hence, in ultimate morphological analysis, the crayfish 

 is a multiple of the histological unit, the nucleated cell. 



What is true of the crayfish, is certainly true of all 

 animals, above the very lowest. And it cannot yet be con- 

 sidered certain that the generalization fails to held good 

 even of the simplest manifestations of animal life ; since 

 recent investigations have demonstrated the presence of 

 a nucleus in organisms in which it had hitherto appeared 

 to be absent. 



However this may be, there is no doubt that in the 

 case of man and of all vertebrated animals, in that 

 of all arthropods, mollusks, echinoderms, worms, and 

 inferior organisms down to the very lowest sponges, the 

 process of morphological analysis yields the same result 

 as in the case of the crayfish. The body is built up of 

 tissues, and the tissues are either obviously composed of 

 nucleated cells ; or, from the presence of nuclei, they 

 may be assumed to be the results of the metamorphosis 

 of such cells ; or they are cuticular structures. 



The essential character of the nucleated cell is that it 

 consists of a protoplasmic substance, one part of uhich 

 differs somewhat in its physical and chemical characters 



