THE CELL 47 



distinct limiting layer, but blends imperceptibly with the rest of the 

 protoplasm, it is known as the crusta. 



In some cells minute channels or canals are present in the cyto- 

 plasm (Fig. 4). These channels may contain branching processes 

 from other cells, forming what is known as a trophospongium. Some 

 intracellular canaliculi are apparently secretory in character and may 

 communicate with fine intercellular secretory channels. In this way 

 the secretion of such cells as the serous cells forming the demilunes 

 of mucous tubules (p. 222), or of the parietal cells of the stomach 

 glands (Fig. 155), is carried out into the duct system of the gland. 

 Other intracellular canaliculi, for example in ganglion cells, commu- 

 nicate with pericellular lymph spaces, thus apparently being con- 

 cerned with the nutrition of the cell. 



Closed networks of canaliculi within the l ■'• ^ X 



cell "apparato reticulare," having no ex- 

 tra cellular communications have also 

 been described. 



2. The Cell Membrane (Fig. i).— In 

 most vegetable cells the cell membrane is 

 the most conspicuous part of the cell and ^*"-^'v^ 

 was responsible for the name "cell" which 



the seventeenth-century botanists, over- , Fig 4.— Intracellular canals 



(trophospongium) of a ganglion 



looking the importance of the enclosed pro- cell (E. Holmgren), 

 toplasm, gave to the little spaces or cavi- 

 ties of w^hich they thought plants composed. A distinct cell mem- 

 brane is present in but few animal cells. An exception is seen in 

 the fat cell where a distinct membrane exists. In most animal 

 cells no membrane has as yet been demonstrated and this has led 

 to a persistent denial of the existence of such a membrane. It is 

 nevertheless probable that most if not all animal cells have some 

 m.odification, however slight, of the periphery of their protoplasm 

 which serves to retain the protoplasm within definite bounds, to pro- 

 tect it as it were and at the same time permit osmosis. When a 

 membrane surrounds the cell, it is known as the pellicula; when cells 

 lie upon the surface, and only the free surface of the cells is covered by 

 a membrane, it is known as the cuticula. 



3. The Nucleus (Fig. i). — This is a vesicular body embedded in 

 the cytoplasm. The typical nucleus, like the typical cell, is sphe- 

 roidal, but the shape of the nucleus varies for dift'erent cells and corre- 

 sponds somewhat to the shape of the cell body, e.g., the rod-shaped 



