PERMANENCE OF THE CHROMOSOMES. 57 



In the fusion of the nuclei of the young ascus the generally parallel 

 position of the strands of the two chromatin systems at the time the 

 centers unite leads naturally to the assumption that the individual strands 

 are combined side by side, so that the number of the chromosomes is 

 not, apparently, doubled in the fusion nucleus. The fusion of the male 

 and female pronuclei probably proceeds in the same fashion, so that 

 here again the chromosome number in the fertilized egg will not appear 

 to be doubled, though the individual chromosomes must be regarded as 

 bivalent structures. The details of this fusion I have not as yet been 

 able to make out, but, since the chromatin of the pronuclei is plainly 

 attached to the centers, just as in the fusion in the ascus, it is probable 

 that the method of combination is the same in both. 



The evidence given above, that the chromosomes are in continuous 

 connection with the central body in the resting-stages, as well as when 

 dividing and fusing, is fairly conclusive that in Phyllactinia, and pre- 

 sumably in other mildews, the chromosomes are permanent cell struc- 

 tures. The facts which favor the doctrine that the chromosomes are 

 everywhere permanent cell organs have accumulated very rapidly and 

 from many sources in recent years. This evidence, both from older 

 and more recent authors, as to permanency of size, number, form, 

 position in the nucleus of the whole series of chromosomes, and the 

 further remarkable facts of chromosome differentiation in size, form, 

 etc., as described by Henking (42), Montgomery (66), and others for 

 the accessory chromosome, and by Sutton (92) for the whole series of 

 chromosomes in Brachystola, has all been fully summarized and its 

 significance critically estimated by Boveri (13). Still more recently 

 Rosenberg (8ia) has described the existence of chromosomes of unequal 

 size in Listera. The vegetative cells here show regularly 10 large and 

 22 smaller chromosomes. 



We have in these newer facts not only proof of specific differences 

 between chromosomes, but indisputable evidence that individual chro- 

 mosomes are perpetuated as such from one cell generation to another. 

 It is a question, however, whether Boveri is justified in combining with 

 the conception of the permanency of the chromosomes as cell structures 

 the further doctrine that they are individual and elementary organisms 

 leading a relatively independent existence in the cell, and thus in a 

 sense comparable in their individuality to the cell itself. It is doubtful 

 whether the facts of permanence in number, form, position in the 

 nucleus, etc., even suggest any such conclusion. 



The conception of the cell as made up in whole or part of more 

 elementary independent organisms is not a necessary conclusion from, 



