12 



NORMAL HISTOLOGY. 



FIG. 2. 



when the cell is without a limiting membrane and composed al- 

 most entirely of active living substance, the outlines are frequently 

 changing, these variations in shape being known as amoeboid 

 movements, from their similarity to the changes observed in the 

 outline of an active amoeba, one of the simplest forms of animal life. 

 As the embryonal cell advances in its life-history, the surrounding 

 conditions to which it is subjected induce, with few exceptions, 

 further specialization. Among the earliest of such effects is the 

 condensation of the peripheral zone of the cell, whereby the reten- 

 tion of a definite form is greatly favored; such peripheral condensa- 

 tion may progress to the production of a distinct limiting membrane 

 the cell- wall. This structure is very frequently wanting; when 

 present, however, it is usually so thin that its optical expression is a 

 single delicate line. The cell-wall is to be regarded as a product of 

 the specialization of a portion of the protoplasm, rather than as an 

 essential part of the cell. 



The adult cell consists of the protoplasm, or cell-contents, possibly 



limited by a cell-wall, en- 

 closing a nucleus, which 

 latter, in turn, often con- 

 tains one or more minute 

 spherical bodies, the nu- 

 cleoli. The more or less 

 definite and characteristic 

 forms which the elements 

 of the various tissues 

 possess on reaching their 

 full development, depend 

 largely upon the changes 

 effected by growth and dif- 

 ferentiation in the proto- 

 plasm during the younger 

 condition of the cells. 



The protoplasm 

 throughout the greater 

 part of the cell usually 

 appears as a finely granu- 

 lar semi-fluid or gelati- 

 nous substance, in which minute granules, the microsomes, as well 

 as other particles of extraneous matters, lie embedded. The most 

 superficial zone of the protoplasm known as the exoplasm is de- 

 void of microsomes. The structure of protoplasm is now recognized 

 as far more complicated than was formerly supposed, comprising a 

 highly elastic and extensible portion the spongioplasm and an 



Typical cell, ovum of cat : a, protoplasm ; b, nucleus ; c, 

 nuclear membrane; d, nucleolus ; e, true cell-wall, closely 

 applied to the surrounding secondary envelope, the zona 

 pellucida. 



