OF THE GERM CELLS OF METAZOA. 209 



taxonomic value ; or (2), that the families of the Heiniptera heteroptera, as they are at 

 present defined, are artificial and not natural groups. I would incline to the latter view, 

 since all our facts would show that chromosomes are very conservative structures, and 

 that the germinal cycle is conservative ; probably the soma may be modified by the 

 action of the environment to considerable extent, before any such action would produce 

 a change in the number of chromosomes. If this standpoint is correct, then the number 

 of the chromosomes would be a very important consideration in deciding the relationship 

 of species ; thus the Coreidce would have to be subdivided into a sub-group with twenty 

 to twenty-four chromosomes (Anasa, Metapodius, Chariesterus), and into one with eleven 

 or twelve chromosomes (Alydus, Corizus, Harmostcs, Protenor}. But it would be a 

 reductio ad absurdum to say that all forms with twelve chromosomes must be related, or 

 all forms with twenty ; the relative boundaries of a family must still be determined from 

 the standpoint of broad comparative anatomy, and then within a group so defined the 

 chromosomal number might be used as a basis for further subdivision. 



(b) Factors Determining the Number. 



A problem of great importance, and one that would seem to lie close to the root of 

 all nuclear phenomena, is that concerning the factors which determine the number of 

 chromosomes. The germ cells of each species have a fixed number of chromosomes, but 

 different species show a very different number, from Ascaris megalocephala univalens 

 with 2 up to Artemia with about 180. What is it that determines this numerical differ- 

 ence? A consideration of the various thinkable factors allows us to limit the problem 

 somewhat, by excluding certain ones which are not real factors. Here we may consider 

 in what relation to chromosomal number stand centrosomes and achromatic spindle 

 elements, number of nucleoli, mass of nucleus and cell body, form of cell, volume and 

 form of chromosomes. 



The size, number and specific peculiarities of the centrosomes seem to have no connec- 

 tion with the number of the chromosomes. The definite number of chromosomes appears 

 in the prophases of mitosis while the nuclear membrane is still intact, and when the cen- 

 trosomes are only commencing to exert an influence upon the other cell constituents. 

 Even the longitudinal splitting of the chromosomes would appear to be an automatic move- 

 ment on their part. The centrosomes may well be centres of movements which produce 

 the separation of the daughter chromosomes, but there appears to be no correlation 

 between the centrosomes and the chromosomal number. And this seems also to be the 

 case with regard to the spindle fibres. Central spindle fibres and polar radiations may 

 vary in their phenomena in different generations of the same species, but the number of 

 the chromosomes remains constant in all generations of the germ cells, for the apparent 



