20 GENERAL ZOOLOGY 



processes. Since the egg and spermatozoon must contain all the 

 hereditary qualities derived from the parents the problem of 

 heredity centers in explaining how an adult organism develops 

 from a cell. 



Unity in Organisms.— In Protozoa, the individual animal is 

 bounded by a single plasma membrane which is comparable to 

 the cell membrane of a metazoan cell. Within the bounds of this 

 membrane there may be one or more nuclei. If a cell be denned 

 as a small mass of protoplasm containing one or more nuclei, 

 then a protozoan is a cell, and the individual organism and the 

 cell are one and the same. The cell in this case is organized as an 

 individual animal. In Metazoa, the animal body is made up of 

 many cells or, the metazoan body may be said to be more or less 

 subdivided by many plasma membranes, represented by the 

 cell walls. Schwann believed that each cell is, within certain 

 limits, an individual, an independent whole; that the vital 

 phenomena of one cell are repeated, entirely or in part, in all 

 other cells of the body, and that the individual cells are not 

 arranged side by side as a mere aggregate, but that they operate 

 to produce a harmonious whole. "The whole organism sub- 

 sists only by means of the reciprocal action of the single elemen- 

 tary units." This idea is in keeping with the fact that the 

 organization of a single cell is capable of maintaining itself in 

 the case of Protozoa; that the metazoan animal starts from a cell 

 and develops as new cells are produced ; and that the individuality 

 or unity of the cell as represented in the fertilized egg may be 

 repeated in each cell formed from the egg. From this point of 

 view the cell is the unit of organization. In Protozoa, the unity 

 of the cell and the unity of the individual organism coincide; 

 in Metazoa the unity of the organism is made up of a number of 

 cell units, cooperating in some way to produce the whole. In 

 either case the cell would be the unit of organization. 



Others have maintained that the fact that the protozoan is a 

 cell should not be permitted to overshadow the equally pertinent 

 fact that the protozoan is also an organism, but an organism 

 whose size is so small that it is possible for it to be contained 

 within a single plasma membrane. The organization of a 

 protozoan is the organization of an individual animal that happens 

 to conform in a general way to a definition of a metazoan cell. 

 If in the case of the metazoan we also regard the organism as the 



