MORPHOLOGICAL SUMMARY. 199 
Thus the body of the crayfish is resolvable, in the first 
place, into a repetition of similar segments, the metameres, 
each of which consists of a somite and two appendages ; 
the 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 hold 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 which 
differs somewhat in its physical and chemical characters 
