ELEMENTS OF STEUCTURE. 65 



46. 



Turning now to the more detailed analysis of cells, the first point to be 

 borne in mind is, that the latter, in the earlier portions of their existence, 

 manifest a certain amount of uniformity, whether as the cells of young 

 embryos or those of later life. Again, that in the course of further 

 development they may assume, as mature and senescent structures, the 

 most diverse shapes, as well as acquire an entirely different body, so that 

 they not unfrequently take on an appearance which may remove them 

 very far, nay, even so far as to be unrecognisable from the plan of a cell, 

 given in the foregoing section. 



1. Directing our attention, then, in the first place, to the size of cells, we 

 find them, in the human body as well as almost everywhere in the animal 

 kingdom, to be within microscopic measurement. The smallest, such as 

 we find, for instance, among the blood-corpuscles, have a diameter of only 

 '006-0 '007 mm. (millimeters), while the largest typical cell of our body, 

 namely, the ovum, may attain a breadth of more than 0'23 mm. Between 

 these extremes the greater number of cells range in diameter from O'Ol 1 

 to '02 3 mm. Those of 0-07-0-115 mm., such as occur, for instance, in fat 

 and nerve tissue, must be looked upon as very large. Thus we observe 

 that the most important structural element of our body is of remarkable 

 minuteness, as usually met with. 



2. If we next turn to the shape of the cell, we are struck likewise with 

 its extreme variability. The fundamental form (fig. 40) is, however, 

 spherical or spheroidal. 



From this primary form two others, easily derived, are produced by 

 compression and flattening in two opposite directions ; these are the 

 flattened and the tall narrow cell. 



Flattened cells springing from the spherical primary form by compres- 

 sion are met with, in the first place (fig. 41), as disks, such as may be 

 seen in human and mammalian blood-corpuscles ; or they may become, 

 by a further increase of superficial extent, flat or scaly structures (fig. 42) 

 such as those, for instance, of the epithelium of many parts of the body. 



c 



Fip. 41 Diskoid cells of human Fig. 42. Flattened scaly epithelium cells 

 blood, a, a, a. At 6, half side ' from the human mouth, 



view ; close by at d, colourless 

 corpuscle. 



That there may exist every gradation between flattened cells and the 

 spheroidal species appears self-evident, and needs no farther comment. 



If the elements in question undergo, on the other hand, lateral com- 

 pression, the resulting form may be either more or less cylindrical or 

 conical, and the tall narrow cell is produced (fig. -43). We shall see later 

 on, in our consideration of the several tissues, that many modifications of 



