MORPHOLOGY 



enter directly into the composition of protoplasm, but are only included within it, 

 are designated by the term Metaplasm. 



The Cytoplasm. — In describing the cytoplasm of the plasmodia of 

 the Myxomycetes and of the walled protoplasts of vegetable cells, 

 mention has been made of a hyaline fundamental substance, the 

 hyaloplasm, which forms a superficial layer on the surface of the cyto- 

 plasm entirely free from granules, while in the interior, as granular 

 plasm, it includes granular matter. The cytoplasm was likewise 

 shown to be a viscous substance, in which internal streaming move- 

 ments of the particles take place, while at the same time its superficial 

 layer of hyaloplasm remains unchanged. In accordance with its 

 viscous fluid character, cytoplasm possesses certain physical peculi- 

 arities. If cells full of protoplasm be opened under water, the out- 

 flowing cytoplasm assumes the form of a drop. 



The cytoplasm in the cells of many Algae has a structure resembling 

 that of foam, while in the higher plants it is no less distinctly fibrillar 

 in structure, and composed of protoplasmic threads. In both cases 

 the chambers or spaces enclosed by the foam-like or thread-like 

 cytoplasm are filled with solution's of various substances. All the 

 granular inclusions lie in the cytoplasm, either in the walls of the 

 cytoplasmic chambers or in the cytoplasmic threads. 



The small granules which are never absent from the granular 

 plasm, and give to it its name, are called microsomes. As they show 

 different chemical reactions, it is inferred that they have also different 

 chemical organisations. Sometimes they appear to be vesicles filled 

 with liquid, and are then termed physodes. In the cells of many 

 Algae such vesicles attain a considerable size, and undergo modifica- 

 tions of their shapes. 



Large vesicles or vacuoles filled with watery solutions are found 

 in the cytoplasm. The cytoplasm separates itself from such vacuoles 

 by means of a protoplasmic membrane or pellicle of hyaloplasm. 

 The sap cavities in the cells of the more highly organised plants are, 

 in this sense, merely large vacuoles. The protoplasmic membranes 

 which surround the vacuoles are particularly tenacious of life ; thus 

 after the other cytoplasm of a cell has been killed with a 10 per cent 

 solution of saltpetre, the walls of the vacuoles will still continue 

 living. As the pressure of the cell sap is controlled by these living 

 vacuolar membranes, H. DE Vries has given them the name tono- 

 plasts. Through the division of the cytoplasm its tonoplasts . may 

 become bisected, and in this way multiply. On the other hand, a 

 single large vacuole may result from the fusion of several smaller 

 ones. It has also been demonstrated by Peeefer that new pellicular 

 membranes may be formed around liquid substances in the cytoplasm. 



The Cell Nucleus. — The nucleus is in all cases fibrillar in struc- 

 ture. It appears to be made up %f threads twisted together and 

 forming an anastomosing network (Fig. 54), which, however, in 



