CHAPTER XIII. 



THE MORPHOLOGY OF TISSUES (HISTOLOGY). 



The bodies of animals can be not only analyzed into organs, 

 but also into tissues and cells. At present the cell is the 

 ultimate unit of structure. That portion of morphology which 

 deals with the microscopic structure of animals, with the forms 

 and arrangements of the cells and of the tissues, is usually 

 called Histology. 



It has been already pointed out that all animals are com- 

 posed of one cell or of many. Among multicellular animals 

 there is always some differentiation of the cells. Thus in the 

 hydra the cells of the endoderm differ in their characters from 

 the cells of the ectoderm. The latter, again, are to be dis- 

 tinguished into the larger muscular cells, and the smaller 

 interstitial cells ; the interstitial cells, again, are differentiated 

 into cnidoblasts, nerve-cells, etc. But the entire animal is 

 quite obviously composed throughout of cells, which are funda- 

 mentally very similar. If the section oi Hydra, on p. 15, be 

 compared with the figure of a transverse section through the 

 body-wall of an earthworm on p. 22, the cellular constitution 

 of the latter will not be quite so obvious. That the epidermis 

 is made up of definite cells of two kinds is as clear as possible ; 

 but underneath the epidermis are two layers of muscles, which 

 are composed of fibres rurming in two directions, and imbedded 

 in a granular substance, through which are scattered nuclei. 

 Nevertheless, these layers are really composed of cells. 



Fig. 76 represents a section through the body- wall of an 



embryo earthworm. It is composed of the same three layers 



as those which are shown in the last figure described. But the 



_ inner layer of muscles is clearly a cellular layer ; it is made up 



