14 HAND-BOOK OF PHYSIOLOGY. 



the rosette or wreath stage (Fig. 7, D) is reached. The loops of the rosette 

 next become divided at the periphery, and their central points become 

 more angular, so that the fibrils, divided into portions of about equal 

 length, are, as it were, doubled at an acute angle, and radiate V-shaped 

 from the centre, forming a star (aster) or wheel (Fig. 7, E), or perhaps 

 from two centres, in which case a double star (diaster) results (Fig. 7, G, 

 H, and i). After remaining almost unchanged for some time, the 

 V-shaped fibres being first re-arranged in the centre, side by side (angle 

 outward), tend to separate into two bundles, which gradually assume posi- 

 tion at either pole. From these groups of fibrils the two nuclei of the 

 new cells are formed (daughter nuclei) (Fig. 7, K), and the changes they 

 pass through before reaching the resting condition are exactly those 

 through which the original nucleus (mother nucleus) has gone, but in a 

 reverse order, viz., the star, the rosette, and the convolution. During 

 or shortly after the formation of the daughter nuclei the cell itself be- 

 comes constricted, and then divides in a line about midway between them. 



Functions of Cells. The functions of cells are almost infinitely varied 

 and make up nearly the whole of Physiology. They will be more appro- 

 priately considered in the chapters treating of the several organs and sys- 

 tems of organs which the cells compose. 



Decay and Death of Cells. There are two chief ways in which the 

 comparatively brief existence of cells is brought to an end. (1) Mechani- 

 cal abrasion, (2) Chemical transformation. 



1. The various epithelia furnish abundant examples of mechanical 

 abrasion. As it approaches the free surface the cell becomes more and 

 more flattened and scaly in form and more horny in consistence, till at 

 length it is simply rubbed off. Hence we find epithelial cells in the 

 mucus of the mouth, intestine, and genito-urinary tract. 



2. In the case of chemical transformation the cell-contents undergo a 

 degeneration which, though it may be pathological, is very often a normal 

 process. 



Thus we have (a.) fatty metamorphosis producing oil-globules in the 

 secretion of milk, fatty degeneration of the muscular fibres of the uterus 

 after the birth of the foetus, and of the cells of the Graafian follicle giving 

 rise to the "corpus luteum." (See chapter on Generation.) 



(b.) Pigmentary degeneration from deposit of pigment, as in the epi- 

 thelium of the air-vesicles of the lungs. 



(c.) Calcareous degeneration which is common in the cells of many 

 cartilages. 



Having thus reviewed the life-history of cells in general, we may now 

 discuss the leading varieties of form which they present. 



In passing, it may be well to point out the main distinctions between 

 animal and vegetable cells. 



