4 INVERTEBRATE MORPHOLOGY. 



category as the vacuoles and other accidental or secondaiy 

 constituents which have been mentioned, and eveiy mass of 

 protoplasm may be considered as consisting of two essential 

 parts, the protoplasm proper or cytoplasm and the special 

 modification of it, the nucleus, which for convenience is 

 termed the caryoplasm. Such a combination of cytoplasm 

 aud caryoplasm forms what is technically known as a cell, and 

 all living organisms are composed of one or more such struc- 

 tures, which are to be regarded therefore as morphological 

 units. 



If the more intimate structure of the cytoplasm of such a 

 unit or cell (Fig. 1) be examined, disregarding the various 



nm 



. 1. DlAGKAM SHOWING THE STRUCTURE OF AN ANIMAL CELL. 



c = centrosome. m = microsome. 



cl cytolymph. nl = nucleolus. 



cr chromatic, nm = nuclear membrane. 



r = reticulum. 



secondary constituents it may enclose, it will be found to 

 consist of a network of exceedingly fine fibrils, along which, 

 and more especially at the points where two or more of them 

 meet, are to be found minute granules which stain deeply with 

 the ordinary microscopical staining reagents. The fibrils 

 constitute the reticulum (Fig. 1, r) of the cytoplasm, and the 

 granules are termed the microsomes (m). The reticulum 

 seems to be formed principally of the proteid substance 

 already mentioned as plastin, and its meshes are occupied by 

 a more fluid substance which has been termed the cyto- 

 lymph (cl). 



