94 BIOLOGY: GENERAL AND MEDICAL 



The cell was originally conceived to be an anatomical 

 unit, and it was supposed that all cells presented more 

 or less uniformity of structure; but it is now known that 

 their structures may be quite dissimilar. Indeed, such 

 latitude now enters into the concept "cell" that it becomes 

 almost impossible to define it. As here used, the term 

 cell signifies a structure capable of displaying all of the 

 vital manifestations, but not capable of resolution into 

 simpler vital structures. 



The cells, thus understood, vary widely among them- 

 selves as to size, morphology, independence, and function. 



In regard to size, there is an enormous difference. 

 Some cells, as the bacteria, are so minute as to be visible 

 only to the highest powers of the microscope, and there 

 is every indication that there are cells so minute that no 

 microscope thus far invented can define them. Other 

 cells, as certain eggs, are large enough to be visible to the 

 naked eye without difficulty. Size is, therefore, an un- 

 important quality of the cell. 



In morphology the cells differ almost as widely as in 

 size. Certain of the most lowly forms of life consist of 

 microscopic specks of protoplasmic jelly in which no 

 definite structure can be made out, so that it has long 

 been controversial whether they are provided with even 

 so important an organ as a nucleus. Other cells, such 

 as the mammalian ovum or egg, are complexly con- 

 structed and provided with many well-known and clearly 

 defined parts. 



Taking the mammalian ovum as a good type for study, 

 we find it conforming to the primitive conception from 

 which the term " cell" was derived; i.e., it is a globular or 

 spherical mass of living substance, shut in by a definite 

 envelope, membrane, or "wall." This primitive idea 

 of all cells being definitely inclosed by a "wall" has long 

 been abandoned, for it is now well known that there are 

 many more cells without than with such a structure. 

 What is true of the cell wall or membrane is true of all 

 the cell structures except the protoplasm and the nucleus. 



