206 KANSAS UNIVERSITY SCIENCE BULLETIN. 



find to be true regarding the development of the sexual charac- 

 teristics which are inherent in every cell of the body, and be- 

 come progressively different in each state of development. 



It is hardly necessary to say that no true student of cellular 

 phenomena regards the chromosomes as the only factor in de- 

 velopment. They play their part in the economy of the cell and 

 accomplish their work solely because they exist under certain 

 conditions. They appear to us as more definite and constant 

 features of cell architecture than any others, and in their be- 

 havior exhibit such indications of initiative and importance as 

 to lead to the belief that theirs is a directive action. Differentia- 

 tion is regarded as a progressively changing series of inter- 

 actions between the chromosomes and other parts of the cell, 

 of such a nature that the nucleus initiates changes which are 

 limited by the conditions of the cytoplasm. 



In order that we may gain some idea of the possible nature of 

 these interactions, I would ask you to consider with me certain 

 phenomena that are observable in the development of the male 

 germ-cells. Studies on spermatogenesis usually begin with a 

 consideration of the spermatogonia, since these are the first 

 cells found in a definite sexual organ, but there is little doubt 

 that these are the last of a line of pure germ-cells that have 

 been set off early in the embryonic history. These spermato- 

 gonia have the paternal and maternal chromosomes present in 

 the same relations as exist at the time of fertilization. They 

 divide rapidly and continuously until they become much de- 

 creased in size and almost the entire cell is nucleus. The cyto- 

 plasm is reduced to a minimum, but the nucleus, at least so far 

 as the chromosomes are concerned, has not been much altered. 

 In the grasshoppers, I feel convinced that the number of these 

 divisions is, for each species, a constant. The reduction of the 

 cytoplasm does not go beyond a certain point, and the process 

 is self-limiting. 



At this point conditions change. The members of the chro- 

 mosome group, instead of remaining separate, as heretofore, 

 unite in pairs, the components of which are size equivalents, 

 and in all probability functional equivalents, from the two 

 parents. This step is one toward which the chromosomes 

 seem to have been tending during the later spermatogonia! 

 divisions, and one which certain elements anticipate in some 

 species. There is every reason to suppose that this synap- 

 sis is a union of homologous elements, and is the consum- 



