OF TITTC OEKM GELL8 OF METAZOA. 



217 



original size, there should be no gradations in volume — no degeneration on their part — so 

 that in a given species we could determine by their number how many times the chromo- 

 somal number had changed. But when new chromatin nucleoli are formed, the older ones 

 would seem to degenerate in the order of their formation. This assumption would explain 

 the occurrence of very minute chromatin nucleoli found in cells of certain Hemiptera, along 

 with much larger ones; the minute ones would represent chromatin nucleoli formed at 

 earlier periods, now on the way to total degeneration and disappearance. We might ex- 

 plain the general occurrence of one pair in the spermatogonia, or of one bivalent one in 

 the spermatocytes, on the conclusion already reached in an earlier part of this paper, that 

 the chromatin nucleoli are metamorphosed for a, special function different from that of the 

 other chromosomes and so necessary for the nuclear activity ; and the reason for their de- 

 generation when new ones are formed , in that a, single pair would generally appear to be 

 sufficient for this function, so that not more; than one pair would remain in functional 

 activity at one time. 



Thus we find that the unexpected discovery of an uneven chromosomal number in 

 the -spermatogonia opens the way to an explanation of certain phenomena, and suggests 

 others not anticipated, [t suggests that there is a gradual evolution in the numbers of 

 chromosomes; that they have not, been fixed from the start, but that with the evolution 

 of the species the chromosomal number changes and at each change probably passes 

 through a period with an uneven number. In the Hemiptera this would seem to be, in 

 the forms examined, a change from higher to lower numbers, and in such a change the 

 odd chromosome becomes metamorphosed — becomes a metamorphosed chromatin nucleolus. 

 If attention be given to these points in other groups of animals, then; can be little doubt 

 that there, too, will be found occasional examples of uneven normal chromosomal num- 

 bers, and probably also in some of these cases the production of structures comparable to 

 the chromatin nucleoli of the Insects. There is great need, first of all, however, to deter- ; 

 mine for the Hemiptera whether in such cases there is a close correspondence between 

 the spermatogenesis and ovogenesis — that correspondence I have assumed, since I have not 

 studied the ovogenesis. 



4. Considerations on the Cycle of the Germ Cells. 



Mere shall he considered in succession some points of broader interest which have 

 arisen in the course of my studies on spermatogenesis. 



(a) The sequence of the stages of the cycle. 



In the germ cells of the Metazoa there may be seen regular cycles of generations 

 following upon one another. In each cycle may be noted a stage of conjugation of ma- 



