44^ ELMER L. SHAFFER. 



brought in from the nurse chamber through the egg-string and 

 assimilated by the cytoplasm; the nucleus exerts a chemical 

 influence (enzyme?) on these products whereby the mitochondria 

 are differentiated in the cytoplasm immediately surrounding the 

 nucleus. According to this view it is possible to explain the 

 presence of mitochondria in all cells (animal and plant) at all 

 times, for all cells are constantly receiving nutriment which is 

 taken into the cytoplasm (assimilated) and then acted upon by 

 the nucleus resulting, among other things, in the elaboration of 

 mitochondria. It thus seems that the mitochondria arise as 

 differentiated parts of the cytoplasm through specific chemical 

 (enzyme) reactions of the nucleus upon the products of assimilation 

 of the cell. What the significance of such cell structures in the 

 cell economy may be is quite another problem. 



E. GENERAL CONSIDERATIONS. 

 i. Chromosomes. 



My study of the chromosomes of Cicada, I believe, presents 

 added evidence to the already large body of facts bearing on the 

 individuality of the chromosomes and my observations indicate 

 a persisting chromosomal organization which is constant through- 

 out the cell-cycle. I have studied the metaphase plates of 

 hundreds of cells, germinal and somatic, and have found no 

 variations in either chromosome number, the relative sizes of 

 the chromosomes or their characteristic grouping. The only 

 exceptions to this is found in the giant spermatocytes and the 

 multinuclear cells in the adhesive gland of the female, where the 

 increase in chromosome number is due to the suppression of the 

 division of the cell-body at mitosis resulting in the formation of 

 polyvalent chromosome groups. In such polyvalent cells, we 

 can still recognize double, triple, quadruple, etc., sets of each of 

 the chromosome pairs. 



There have appeared at various times in the cytological 

 literature discrepancies of chromosome numbers in certain species, 

 purporting to show that chromosomes vary in number and cannot 

 be regarded as persistent structures of the cell. McClung ('17) 

 has dealt ably and at length with such criticisms and has par- 

 ticularly concerned himself with the work of Delle Valle. Ac- 



