THE DIPLOID CHROMOSOME COMPLEXES OF THE PIG 195 



It may be that this breaking up of the chromosomes is corre- 

 lated with the genetic impurity of the plant. The mosquito is 

 without doubt a 'pure' form, and its somatic chromosomes are 

 constant in number. It is difficult to see how Ascaris can be 

 anything but a homozygous form, and yet the somatic chromo- 

 somes are highly fragmented. The pig is probably not pure, 

 considering the length of time it has been domesticated, and its 

 somatic chromosomes show a variation in number. The evi- 

 dence would seem to be rather evenly balanced at present and 

 no conclusions can be safely drawn. 



I think, however, that it may be said with some assurance 

 that the present work, and such previous work as is reliable, has 

 tended to show that the chromosome equipment is the same in 

 the various cells of the body, and when fragmentation occurs, it 

 has not been found to affect one tissue more' severely than an- 

 other. In other words, no tissue specificity of the chromosomes 

 has been found. 



The fact that the chromosomes of the majority of somatic 

 cells are more or less fragmented would lend some support to 

 the theory that a germ line or cycle exists, the cells composing 

 which are set off at an early period in the history of the indi- 

 vidual. While it is not impossible (from the evidence of the 

 present work) that the germ cells may come from already special- 

 ized tissues (Hargitt, '17), it seems rather improbable that cells 

 in which the chromosomes, in all probability, have broken up 

 should be able to reform the typical germinal complex. In view 

 of the present work it seems very likely that the anlagen of the 

 future reproductive cells are set off or guarded from the pro- 

 cesses ^Vhich are taking place in the soma, early in the life his- 

 tory of the animal. Otherwise, since, in the majority of somatic 

 cells studied, the chromosomes break up to a greater or lesser 

 degree, we should have to postulate, either that the chromosomes 

 have the power to recombine as stated above, or that certain 

 cells of some tissue do not undergo the changes common to the 

 rest of the soma. The latter supposition is essentially the 

 germ cycle theory. 



