24 



CELLS AND EXTRACELLULAR ELEMENTS. 



FIG. 1. 



Diagrammatic representation of an am- 

 reboid cell, showing the nucleus, nu- 

 clear reticulum, nucleolus, the cen- 

 trosome and attraction-sphere, the 

 spongioplasm, hyaloplasm, and pseu- 

 dopodia. 



some is an important feature of most cells. Many cells are 

 surrounded by a cell-wall, while some have processes and other 

 special features. 



The cell-body, or cytosome : As a general thing, the body or 



main (non-nuclear) portion of 

 cells is composed of typical 

 and active protoplasm. The 

 protoplasm of cell-bodies is 

 often called cytoplasm. In typ- 

 ical cells, like the leukocyte, 

 two parts can be distinguished 

 in the protoplasm : the hyalo- 

 plasm, a clear, semifluid, motile 

 portion similar in appearance 

 to the white of an egg ; and the 

 granuloplasm, or spongioplasm., 

 which in the living cell presents 

 a granular appearance, but is 

 supposed by many to be made 

 up of fibres or a sponge-like 

 reticulum. The granular portion usually occupies the central 

 portion of the cell, when it is called the endoplasm, or endo- 

 sarc ; the hyaline portion then forms the outer part of the 

 cell, and is called the ectoplasm, or ectosarc. 



Sometimes the protoplasm contains granules, as of pigment, 

 or foreign particles which have been taken into the soft sub- 

 stance of the cell from without, as bacteria and other bodies 

 absorbed by leukocytes, or food- particles taken in by the 

 small unicellular organisms. Some cells contain vacuoles, 

 minute rounded cavities or spaces ; in some of the protozoa 

 these are contractile, being alternately distended with fluid 

 and then emptied by the contraction of the surrounding pro- 

 toplasm, thus accomplishing some sort of circulatory process; 

 certain human cells are vacuolated. Cells often contain sub- 

 stances elaborated by their own activity and stored up in 

 their bodies in granules or small masses. One of the com- 

 monest of these is fat ; cells, as in the liver, secreting mam- 

 mary gland, the vitelhis of the ovum, often contain small 

 fatty globules. In some cases, as in adipose tissue, this proc- 

 ess is carried to an extreme degree and the cell becomes com- 



