14 CELLS AND EPITHELIAL MEMBRANES. 



tion ; these constitute the fifth class, and are known 

 as ciliated cells. (PL I. 'fig. VII.) 



Fusiform ce'.is. Fusiform cells are found principally in tissues of 

 recent origin which are undergoing fibrous transfor- 

 mation ; cicatrices are formed by this class of cells, 

 and some tumors are composed almost entirely of 

 them. (PI. IV. fig. IV.) 



^he ^ as ^ c ^ ass com P r i ses those cells in which the 

 cell wall is developed into tubular or filiform prolon- 

 gations or branches. Examples : Most nerve cells of 

 the nervous centres and ganglia ; the cells of the ex- 

 ternal surface of the choroid ; bone cells, plasmatic 

 cells, etc. (PL XIII. fig. I., fig. II., PL V. fig. IV., PL 

 III. fig. IV.) 



Every cell must derive its origin from another pre- 

 viously existing cell. In the present state of science 

 but two modes are known in which cell generation is 

 accomplished in human histology: endogenous gene- 

 ration, and multiplication by cleavage. 



In endogenous generation the process is not always 

 exactly the same ; sometimes the nucleus of the pri- 

 mitive cell developes itself into two secondary nuclei, 

 each of which on the disappearance of their common 

 envelope, surrounds itself by a portion of the granular 

 contents of the cell, and a new cell wall making its 

 appearance on its surface, (Kolliker), the infant cell is 

 thus completed; (segmentation of the yelk). In 

 other cases the young nuclei, instead of making their 

 appearance first in the interior of the nucleus of the 

 primitive cell, develope themselves directly from the 

 granular contents, and then grow into perfect cells in 

 the mode already described ; e.g. cells of foetal mar- 

 row. (PL VI. fig. IV.) 



