6 HISTOLOGY. 



body, the centrlole. Such centrosomes have been observed in the in- 

 vertebrate animals. The cells of vertebrates are not regarded as favorable 

 for investigations of the finer structure of centrosomes. In them generally 

 both centroplasm and centriole appear as a single small granule, the 

 centrosome. This granule is usually, but not always, surrounded by a 

 zone of protoplasm which is so modified as to form a darker or a lighter 

 area, the archo plasm (Fig. i). (The archoplasm of certain spermatic cells 

 is called the idiozome.} The centrosome may be near the nucleus or 

 distant from it, frequently being found between the nucleus and the free 

 surface of the cell. Rarely, as in a few invertebrates and in cancer cells, 

 the centrosome has been found within the nucleus. In many gland cells 

 it lies where the secretion accumulates, the expulsion of which is accom- 

 plished by the contraction of the protoplasmic framework between the 

 masses of secretion. In the intestinal epithelial cells which send out 

 motile projections of protoplasm (pseudopodia), the centrosome lies just 

 beneath the place of origin of these projections. If one considers also the 

 relation of the centrosome in the spermatozoa as well as its role in cell 

 division, it seems almost certain that the centrosome is the active or passive 

 center of the motor functions. In connection with cell division, the 

 centrosome undergoes a cycle of changes of varying duration. That 

 stage which is continued longest is characterized by a doubling of the 

 centrosome, following the division of the centriole in two. The double 

 body thus formed is the diplosome. In many resting cells, or those not 

 actually in the process of division, a diplosome is found, and this is signifi- 

 cant as indicating the readiness of the cell for undergoing division without 

 delay. 



CELL WALL. 



A cell wall or cell membrane is an independent membranous layer 

 covering a cell and being clearly distinct from the underlying protoplasm. 

 It is not an essential constituent of a cell. Often it is lacking, and when 

 present it is either a modification or a secretion of the peripheral protoplasm. 

 If the membrane surrounds the cell on all sides it is called a pellicula; 

 if it is on only one side, covering the free surface, it is a cuticula. (The 

 former term is seldom used.) Cells may unite with one another by proto- 

 plasmic processes of varying length and width, thus forming cellular 

 networks; or they may completely fuse so that their nuclei appear irregu- 

 larly distributed through a single mass of protoplasm. Such a formation 

 is a syncytium [plasmodium]. This name is applied also to such structures 

 as the striated muscle fiber, due not to the fusion of cells but to the multi - 

 plication of nuclei in an undivided mass of protoplasm. 



