The Cellular Basis 129 



and cytoplasm, such as the chromosomes, chromomeres, centro- 

 somes, etc. In all cases cells come from cells, nuclei from nuclei, 

 chromosomes from chromosomes, centrosomes from centro- 

 somes, etc. 



Indeed, the manner in which all living matter grows indicates 

 that every minute particle of protoplasm has this power of taking 

 in food substance and of dividing into two particles when it has 

 grown to maximum size; this is known as panmerism. Presum- 

 ably this power of assimilation, growth and division is possessed 

 by particles of protoplasm which are invisible with the highest 

 powers of our microscopes, though it is probable that these par- 

 ticles are much larger than the largest molecules known to chem- 

 istry. The smallest particle which can be seen with the most 

 powerful microscope in ordinary light is about 250 //./A (millionths 

 of a millimeter) in diameter. The largest molecules are prob- 

 ably about 10 /iju, in diameter. Between these molecules and the 

 just visible particles of protoplasm there may be other units of 

 organization. These hypothetical particles of protoplasm have 

 been supposed by many authors to be the ultimate units of assimi- 

 lation, growth and division, and in so far as these units are sup- 

 posed to be the differential causes of hereditary characters, they 

 are known as inheritance units. 



Inheritance Units. It is assumed in practically all theories of 

 heredity that the "inheritance material," or the germinal proto- 

 plasm, is composed of ultra-microscopical inheritance units which 

 have the power of individual growth and division and which are 

 capable of undergoing many combinations and dissociations dur- 

 ing the course of development, by which combinations and 

 dissociations they are transformed into the structures of the 

 adult. Various names have been given to these units by 

 different authors; they are the "physiological units" of Herbert 

 Spencer, the "gemmules" of Darwin, the "plastidules" of Els- 

 berg and Haeckel, the "pangenes" of de Vries, the "plasomes" of 

 Wiesner, the "idioblasts" of Hertwig, the "biophores" and "de- 

 terminants" of Weismann. 



