190 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
6, 7. The ova and the spermatozoa have already been 
described (pp. 182—185). 
It will be observed that the blood corpuscles, the 
epithelial tissues, the ganglionic corpuscles, the ova 
and the spermatozoa, are all demonstrably nucleated 
cells, more or less modified. The first form of con- 
nective tissue is so similar to epithelial tissue, that it may 
obviously be regarded as an aggregate of as many cells as 
it presents nuclei, the matrix representing the more or 
less modified and confluent bodies of the cells, or products 
of these. But if this be so, then the second and third 
forms have a similar composition, except so far as the 
matrix of the cells has become fibrillated, or vacuolated, 
or marked off into masses corresponding with the several 
nuclei. By a parity of reasoning, muscular tissue may 
also be considered a cell aggregate, in which the inter- 
nuclear substance has become converted into striated 
muscle; while, in the nerve fibres, a like process of 
metamorphosis may have given rise to the pellucid 
gelatinous nerve substance. But, if we accept the 
conclusions thus suggested by the comparison of the 
various tissues with one another, it follows that every 
histological element, which has now been mentioned, 
is either a simple nucleated cell, a modified nucleated 
cell, or a more or less modified cell aggregate. In 
other words, every tissue is resolvable into nucleated 
cells. 
