THE PROTOPLASMIC SYSTEM 



the full complement of living functions. One can cut off 

 pieces of a single-cell animal; if these pieces be without 

 nuclei they die, whereas a piece containing the nucleus or 

 pieces containing portions of the nucleus are capable of 

 growth to the full size of the normal creature. In turn, 

 nuclei deprived of their cytoplasm do not live. The unit of 

 living matter in these cases is thus shown to reside in the 

 nucleo-cytoplasmic organization. 



We therefore adopt the point of view that the proto- 

 plasmic system is the unit of life. For the majority of 

 animals and for all their eggs the protoplasmic system 

 reveals itself as comprised of a single nucleus enclosed by 

 cytoplasm — ground-substance containing suspended mat- 

 ter, the cytoplasmic inclusions — and the whole enclosed 

 by a membrane. Such cells exhibit variations in several 

 directions. 



Cells may show extreme size-variations. They range 

 in size from one micron in length, discernible only under the 

 highest microscopical magnification, to the sporozoon 

 parasite found in the digestive tract of Crustacea which 

 measures i6 mm. in length, or to the unfertilized egg of the 

 ostrich, 105 mm. in diameter, or that of a shark, 220 mm. 

 in diameter, or to the nerve-cells in the human spinal cord, 

 which may be one meter in length. Organization of the 

 cell thus does not depend upon size. We may imagine that 

 the skein of life is greatly contracted in some while it is 

 more diffuse in others. The maintenance of the cell-size 

 that is characteristic of a species still remains an unsettled 

 problem. Maintenance of a definite and specific size is a 

 revelation of the self-regulative capacity of the proto- 

 plasmic system. 



Cells also show most varied shapes and forms; some are 

 almost perfect geometrical figures — spheres, cylinders, 

 cones; others are irregular or of forms not easily reducible 

 to conventional geometrical figures. Often when in groups 



37 



