208 KANSAS UNIVERSITY SCIENCE BULLETIN. 



siclerable, and is accompanied, or rather preceded, by a corre- 

 sponding nuclear enlargement. This growth in the case of the 

 ovum is much more extensive, including the formation of all 

 the varieties of differentiated "stuffs," but takes place under 

 similar conditions. In both sexes this unique state of the cell 

 is terminated by two mitoses, one of which witnesses the sep- 

 aration of the paired homologous chromosomes and their dis- 

 tribution into different cells. This, in reality, is the beginning 

 of a new individual, for here are new conditions throughout the 

 cell. It seems possible to me, in the light of our present knowl- 

 edge of animal development, to gain some idea of the general 

 nature of the phenomena involved in maturation and fertiliza- 

 tion, and to grasp something of the meaning of the cell changes 

 that take place at these periods. Clearly enough, in matura- 

 tion there is a separation of chromosomes and in fertilization a 

 restoration to the normal number. These alternately joined 

 and separated chromosome groups are of different sexual or- 

 igin and distribution, but are themselves without sexual char- 

 acters, for in one generation they may be in a male organism 

 and in the next in a female. 



But when we have recorded these important facts we have 

 by no means exhausted our knowledge of the difference between 

 the immature and mature germ-cell. Much important work 

 has been done within recent years upon the organization of the 

 egg, and it has been clearly demonstrated that this is of a high 

 order of complexity. The recognition of different "stuffs" in 

 the egg, and the discovery that they are of various organ-form- 

 ing powers, show most clearly that our theories of development 

 must take the cytoplasm into due consideration. Such a recog- 

 nition awarded cytoplasmic localization lessens in no measure 

 the importance of the chromosomes, a fact that has been appre- 

 ciated by none more clearly than by those who are most fa- 

 miliar with egg organization. Experimentally it has been 

 proved that these particular materials are able to develop the 

 earlier stages of the embryo without cell formation or nuclear 

 division ; and yet those who have most knowledge of such pro- 

 cesses do not minimize the importance of the chromosomes, 

 even though it may be granted that the cytoplasm has much to 

 do with conditioning the early stages of development. Nor 

 does the additional fact that the final stages of somatic differ- 

 entiation are characterized by the preeminence of the cyto- 

 plasm weaken our belief in the primary importance of the chro- 



