1()2 EDWIN G. CONKLIN 



The determination of tlie number of neuroblasts occurs so early in 

 the history of an individual, and under such uniform conditions, that it 

 is very tlifficult to regard the environment as possessed of much power 

 to cause variation in this r(\spect, and for this reason among members 

 of the same race a high degree of constancy in this character is to be 

 anticipated. 



He regards differences in brain weight as due to differences in 

 the size of the nerve cells, the number remaining constant. Hard- 

 esty ('02) found that in various mammals the motor nerve cells 

 from the spinal cord were largest in the elephant and smallest in 

 the mouse, while in animals which were intermediate in size the 

 nerve cells were intermediate. Levi ('05) has made an extensive 

 study of cell size in different species, particularly mammals. 

 He finds that the size variations of epithelial and gland cells are 

 insignificant and are not correlated with body size. Measure- 

 ments of ganglion cells gave entirely different results; here the 

 size of the cell varies with the size of the animal. Nerve fibers 

 and lens fibers show the same correlation with body size as do 

 ganglion cells. In muscle he finds that the diameter of the fiber 

 is usually larger in large animals than in small ones, though this 

 is subject to many variations. Levi points out the significant 

 fact that epithelial and gland cells divide throughout life, whereas 

 the other types of cells named cease to divide at an early age. 



Morgan ('95) concluded that in echinid larvae derived from 

 isolated blastomeres the number of cells is approximately pro- 

 portional to the size of the blastomeres or egg fragments from 

 which they came. However in the case of partial larvae of Am- 

 phioxus Morgan ('96) held that one-half larvae contained about 

 two-thirds as many cells as whole larvae, while one-fourth larvae 

 had more than one quarter, but not quite one-half the total num- 

 ber in a whole larva. Driesch ('98, '00) determined in a very 

 satisfactory manner that the number of cells in partial larvae 

 of echinids is one-half the normal number in one-half larvae, one- 

 fourth in one-quarter larvae, etc., while in larvae from two fused 

 eggs the number of cells is double the normal number. And since 

 these partial or double larvae all have the typical structure of a 

 normal larva he was led to the formulation of the 'rule of the 

 fixed size of specific organ-cells.' 



