THE TYPICAL CELL * 



smaller, until the whole cell finally occupies the position of its 

 former process. The cell has thus changed its position locomo- 

 tion has heen accomplished. 



(b) Ciliary Motion. This is a rapid waving motion of fine 

 hair-like cilia which project from the free border of certain cells, 

 as, for example, the ciliated epithelium of the respiratory tract. 

 The rapid undulatory vibrations of the flagellum attached to cer- 

 tain other cells e. g., the spermatozoa is closely allied to ciliary 

 motion. The ciliary vibrations are exceedingly rapid, occurring 

 many times to the second. 



(c) Molecular Motion. This is a peculiar dancing movement 

 of the finer granules, which occur within cell protoplasm. These 

 granules may be microsomes, various forms of paraplasm, pigment 

 granules, etc. A closely allied form is pigmentary motion in 

 which pigment granules, which are at first equally distributed 

 throughout the cell, are collected into a group, which usually sur- 

 rounds the nucleus ; the reverse then occurs, the pigment granules 

 becoming again equally distributed through the cell protoplasm. 

 Molecular motion is readily 'observed in the pigment granules 

 of the plasmodium malariae, a parasite occurring in the blood of 

 persons afflicted with malarial fever. Molecular motion is closely 

 simulated by Brownian motion, a peculiar dancing movement 

 occurring when fine granular particles of inert substance are sus- 

 pended in a fluid of nearly equal density. 



Secretion, Changes in the appearance of cells may be due to 

 secretory activity. Thus, during rest, glandular cells become dis- 

 tended with their secretion ; they appear swollen, and their nuclei 

 are obscured and pushed toward the attached margin of the cell. 

 During activity secreting cells become shrunken and regain their 

 ordinary protoplasmic appearance; their nuclei again approach 

 the center, or even the free margin of the cell. 



In other types of secreting gland, as in the sebaceous glands, the 

 secretion is produced by a disintegration of the cell protoplasm, 

 which, in most of these cases, undergoes a fatty metamorphosis. 



Growth. This process involves changes in the size, shape, and 

 consistence of the cell. The increase in size in most cells is not 

 marked. It is, however, frequently sufficient to produce an in- 

 creased pressure upon surrounding cells, which is to a certain 

 extent accountable for the varying shapes assumed by those cells 

 which are closely packed within the organs of the body. Most 

 cells in their early embryological condition are nearly spherical in 



