THE CELL DOCTRINE. 81 



bryo cells resulting from the segmentation of the ovum, 

 as typical cells, which are thus composed of proto- 

 plasm surrounding a nucleus, which nucleus, as well 

 as protoplasm, are products of like constituent parts 

 of another similar cell. "The cell leads in itself an 

 independent life, of which the protoplasm is espe- 

 cially the seat, although to the nucleus also undoubt- 

 edly falls a most important, though not yet precisely 

 determined role. Protoplasm is for the most part 

 no further distinct than that it will not commingle 

 with the surrounding medium, and in the peculiarity 

 that with the nucleus it forms a unit. Upon the 

 surface of the. protoplasm, there may form a mem- 

 brane, which, although derived from it, may he chemi- 

 cally different, and the assertion that it is the begin- 

 ning of a retrogression may be defended. A cell with 

 a membrane cannot divide .itself, unless the proto- 

 plasm within the membrane divides itself. A cell 

 within a membrane chemically different from proto- 

 plasm, is like an encysted infusorium."* 



Briickef went even further in his definition, and 

 said that it was not shown that the nucleus even is 

 an essential element of the cell. In proof of which 

 he adduces the cells of cryptogams and says: "We 

 have no positive knowledge either of the origin or 

 function of the nucleus, and, indeed, the constancy 

 of its occurrence seems subject to certain limitations 

 if we take into consideration the ceils of cryptogams, 



» Schultze, Max, Protopl. d. Bhizopoden. Leipzig, 1863. 

 j- Bi'iiclte, B., Die Elementar-orgiinisraen, p. 18-22. 1861. 



