190 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



6, 7. The ova and the spermatozoa have already been 

 described (pp. 132135). 



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. 



