HEREDITY 231 



agencies were called " factors " : thus there were factors for the 

 greenness of the peas, the yellowness of the peas, the tallness of 

 the plants, and so on. Later on there were supposed to be single 

 or double *' doses " of a factor, there were *' enabling," or qualify- 

 ing factors, etc. Factors were said to be coupled and so on. 

 When the appearance of a character could not be " explained " 

 by a single factor such subsidiary hypotheses were made. Such 

 results as we have considered came from actual breeding experi- 

 ments, but in their interpretation the factorial hypotheses were 

 made. Now from what has already been said as to the nuclear 

 phenomena in germ-cells it will be seen to have been inevitable 

 that the results of breeding experiments should have been associ- 

 ated with the results of cytological investigation of germ-cells. 

 And, after Weismannism, it was inevitable that particulate things, 

 or agencies, in the chromatin of the germ-cells should have been 

 identified with the Mendelian factors. Thus instead of factors 

 we now speak of " genes " and these are, or are associated with, 

 units of chromatic, nuclear substance. We must now consider 

 what appear to be the results of cytological investigation that are 

 relevant in this respect. 



81^. The Maturation of the Germ-cells. The cells of 

 the gonads that are going to become ova and spermatozoa are 

 generally regarded as the descendants of original cells in the 

 embryonic gonad-anlagen that have persisted into adult life in 

 the undifferentiated state. (But it may be that peritoneal cells 

 of the adult body may also become germ-cells.) Either a relatively 

 small number of gonidial cells (in the late embryo) serve throughout 

 life as the cells that are going to become ova, or, every year, these 

 cells proliferate (or reproduce) so as to become millions (perhaps) 

 of ova that are annually " spawned." In all cases there is a 

 continuous (or annually recurring) proliferation of cells that are 

 going to become spermatozoa. 



Before the gonidial cells become ova or spermatozoa they 

 undergo " maturation." Each cell has a certain number, A^, 

 of chromosomes. Each gonidial cell may enlarge and changes 

 occur in its chromosomes. The latter may bunch up, extend 

 out into a long thread, disintegrate into granules, etc. In spite 

 of their disappearances and reappearances there is said to be a 

 " continuity of the chromosomes." This is necessary to genetic 

 hypotheses, but since the chromosomes are actually nucleo-protein, 



