HISTOLOGY OF THE FROG 79 



as being nearly synonymous, the former having a slightly 

 wider and more general meaning than the latter. 



The ultimate structure of protoplasm, as far as it can be 

 determined by the best modern microscopes, is comparatively 

 simple and gives no clue to the wonderful vital properties 

 manifested by living cells. The cytoplasm of an undifferenti- 

 ated cell, such as a leucocyte, appears as a network, or rather 

 a sponge-work, of a denser albuminous material holding a more 

 fluid substance in its meshes or cavities. The granules so 

 generally present in the cytoplasm are of various kinds ; they 

 may simply be substances taken into or elaborated by the cell; 

 or they may be proteid bodies of more complex constitution, 

 multiplying by division and behaving like miniature cells 

 within the cell. In the latter case they are distinguished as 

 plastids. The nucleus has a more complicated visible structure 

 and chemical constitution than the cytoplasm, as is shown by 

 its behaviour towards staining fluids and other reagents. Ex- 

 ternally, separating the nuclear contents from the surrounding 

 cytoplasm is a nuclear membrane. Within the membrane is 

 a network or sponge-work of fine fibrils, composed of a sub- 

 stance called linin, holding a semi-fluid matter in its meshes. 

 As the fibrillar network does not stain readily with the usual 

 dyes it is known as the achromatic network. The most obvious 

 and in some respects the most important constituent of the 

 nucleus is a deeply staining substance called chromatin which, 

 in the normal resting condition of the cell, is scattered in the 

 form of granules along the achromatic network and therefore 

 appears itself to have a reticular arrangement. The nucleus 

 may also contain a spherical body, the nucleolus, which stains 

 as readily as the chromatin but in a different manner, and is 

 composed of a chemically different substance called plastin. 

 Chromatin is a compound of a highly phosphorised organic 

 acid nucleic acid with albumen. The cell, then, has an 

 organisation, a complex constitution, such as Dujardin postu- 

 lated for his sarcode. It consists essentially of a cell-body or 

 cytoplasm and a nucleus, and both cytoplasm and nucleus are 

 mixtures of several very complex chemical compounds whose 

 formulae have not yet been worked out. 



It is clear that both kinds of corpuscles contained in the 

 frog's blood, the haematids and the leucocytes, are cells. Both 

 have a cell-body and a nucleus, yet they differ in a marked 



