Since we are highly ignorant of the system, a cell becomes very difficult 

 to define in anything but descriptive terms. What we commonly think of 

 as cells consist at least of a mass of protoplasm surrounded by a mem- 

 brane and containing a nucleus. Specific types of cells may have, and in 

 most cases apparently must have, other contents of obviously organized 

 nature. Even this skeletal definition leads us into a certain amount of 

 trouble. We must, for instance, assume either that coenocytic organisms 

 and structures are single cells regardless of the number of nuclei or that 

 a membrane is essential only where a cell is in contact with nonproto- 

 plasmic material. As noted later, even some coenocytic cells appear to 

 be compartmentalized by membranes visible only with the electron 

 microscope. We may avoid this difficulty by defining a cell as a nucleus 

 and that portion of the surrounding protoplasm upon which it exerts a 

 major effect. This may seem to imply that the nucleus is the real essential 

 of the cell, which of course is probably untrue, although in the case of 

 viruses we may be approaching the situation where a nucleus or part 

 thereof exists in the presence of a minimum or none of its own cyto- 

 plasm, for example, the bacteriophage. In general, however, it would 

 seem that both parts of the system are essential and interactive. Nonethe- 

 less, most evidence indicates that the nucleus is essentially a control 

 center or seat of organization. Enucleate protoplasts either fail to carry 

 on life processes at all or do so only temporarily and then in an imperfect 

 fashion. Decytoplasmated nuclei, however, are even more inadequate. 



The historical development of the science of cytology is of consider- 

 able interest, and some knowledge of it is almost essential in order to 

 appreciate its present problems and direction. In the introduction to the 

 third edition (1925) of 'The Cell," E. B. Wilson gives a brief but rather 

 complete sketch. A more up-to-date interpretive discussion is found in 

 Schrader (1953) and a general history of the science has been provided 

 by Hughes (1959). Since there is little that can be added to these excel- 

 lent accounts, only a very brief note of the more important develop- 

 mental stages will be offered here. 



Although M. J. Schleiden (1838) and Theodore Schwann (1839) are 

 generally credited as the initiators of the cell theory, this is not altogether 

 fair. It does seem to be true that they were the first to assemble con- 

 vincing evidence that both plant and animal tissues are made up of 

 aggregations of cellular units and they were perhaps also the first to 

 attach a meaning to the term "cell" which is acceptable today. Schleiden, 

 however, was wrong on many details, especially in regard to his notions 

 concerning cellular replication. His main theory in this regard was, 

 briefly, that new cells were formed inside of old ones from the nucleus 



2 / CHAPTER 1 



