CHAXGES OF CELLS IX THE FORMATIOX OF TISSUES. II 



Changes which occur in Cells. — The changes which may occur 

 in cells in relation to the production of the textures are of two prin- 

 cipal kinds, according as the form of the cell, or the nature of the sub- 

 stance composing it, undergoes alteration. These changes may occur at 

 one and the same time — indeed, this is commonly found to be the case ; 

 it will, however, be more convenient here to consider them separately. 



1. Chemical and Plastic Changes occurring in Cells. — The protoplasm 

 originally composing the embryonic cell may become variously altered in 

 chemical constitution, all such changes tending to diminish the original 

 activity of the cell and to fit it for a special function. An alteration com- 

 monly met with in older cells is the conversion of the outer portion of 

 the protoplasm into a comparatively dense layer, which constitutes an 

 investment for the remainder, and in this way approximates the cell 

 more to the vegetable type. Such a transformation is met with in a 

 high degree in the stratified epithelia, in which the cells of the upper- 

 most layers become almost entirely transformed into dense horny scales. 



Another change which is apt to occur is the deposition within the 

 cell of various chemical principles, which are either derived directly 

 from the plasma of the blood, in which in such cases they pre-exist, or 

 are elaborated by the cell itself from some other constituent of that 

 fluid. Examples of these changes are to be found in the deposit of fat 

 and pigment, and of tiie peculiar constituents of certain secretions 

 within the cells of the tissue or gland producing them. 



The deposition of fat occurs ordinarily and in its most characteristic fonn in 

 the coipuscles of the connective tissue, transforming them into fat cells, although 

 it may occasionally be found in other cells, such as those of the liver and of 

 cartilage. Pigment on the other hand may be deposited both in connective tissue 

 cells and in epithelium, and this to such an extent as to give an intensely black 

 appearance to the part, as in the choroid coat of the eye and in the cuticle of the 

 negro. 



Sometimes these chemical changes are accompanied by others of 

 a plastic or organizing character, as in the fibrillation which is often 

 found to occur in cells, and notably in those of the nervous anvl 

 muscular tissues, as well as in the formation of the spontaneously 

 moving bodies called spermatozoa in the spermatic cells. Another 

 example of such a change is to be found in the formation of red blood 

 corpuscles within the cells of connective tissue. 



These plastic changes are equally unexplained with the other alterations of 

 form and stmctuie which accompany the production and metamoiphoses of cells. 

 As regards the changes in the quantity and chemical natui-e of the contained 

 matter, it may be remarked that the introduction of new matter into a cell is to 

 a gi-eat extent a phenomenon, of imbibition. In addition to this, many cells, by 

 vu-tue of their amcoboid movements to be presently described, are enabled to take 

 into then- substance minute solid particles, both inorganic and organic. But. 

 while an alteration in the contents of a cell may be thus brought about by 

 imbibition and intersusception of pre-existing material, the contained substance 

 may also be changed in its qualities by a process of conversion or elaboration 

 taking place within the cell. 



2. Changes in Form — The changes of form which may occur in 

 cells are of two kinds, the one being merely passive and mechanical, 

 the other dependent upon the growth of the cell. Instances of the 

 former are seen in those cases where, by mutual compression, the cells 



