160 EDWIN G. CONKLIN 



certain that it is associated not merely with growth in size but 

 also with growth in complexity. The degree of differentiation 

 of an organism is not determined entirely by the number of cells 

 in its body, but cell number is certainly one of the factors in 

 differentiation. It is now known that the protoplasm of certain 

 egg cells is composed of different substances which may be 

 localized in different parts of the egg, and in this respect it 

 resembles the protoplasm of a protozoan cell. Lillie ('02) has 

 shown that certain simple differentiations, such as the appearance 

 of cilia, may occur in eggs which do not undergo cleavage, but 

 these differentiations do not go beyond this primitive, pro- 

 tozoan stage. When these differentiated portions of the egg 

 protoplasm are separated from one another by cell membranes, 

 the differentiations increase in degree, if not in number. The 

 whole progress of embryonic differentiation is thus associated 

 with differential cell division. 



Of course there are many cell divisions which are non-differ- 

 ential, in which the daughter cells are alike, and such cell divisions 

 are not essential to progressive differentiation, though they are 

 essential to growth. We are thus able to separate growth and 

 differentiation, for while the latter consists in an increase in the 

 kind of structures, the former consists in an increase in their size 

 and number merely. In embryonic development, however, these 

 two processes usually go hand in hand ; and in many instances size 

 differences between cells are the earliest differentiation which 

 can be recognized. 



Almost all species of animals and plants have a more or less 

 characteristic body size which, in spite of individual variations, 

 may be said to constitute the norm of the species. Specific differ- 

 ences in body size may be slight, or they may be enormous, as 

 in the case of the mouse and the elephant. In similar manner 

 different individuals of the same species may differ in a marked 

 degree in body size, the individuals which represent the extremes 

 of size being known as dwarfs and giants. The question whether 

 the difference between a dwarf and a giant, or between a small 

 sized and a large sized race or species, is due to a difference in 

 size of the ultimate units of structure, the cells, or to a difference 



