6O SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



nucleus in the resting condition. The nucleus lies in the cytoplasm as 

 a ball pushed between the organic rays, none of which are connected 

 with it. Heidenhain (41, p. 504-505) avowedly leaves untouched the 

 question as to how the spindle fibers become attached to the chromo- 

 somes, but is positive that there is no fixed relation of position or con- 

 nection between the nucleus and center in the resting condition. He 

 plainly feels the weakness of his position on this point, and resorts to 

 a doubtful analogy in pointing out that the discontinuity of nucleus and 

 center in the resting condition, which passes over into a continuity in 

 mitosis, is no more difficult to understand than that independent cells, 

 i. e., muscle and nerve, may become connected in ontogeny. 



Heidenhain's mechanical theory thus breaks down at a critical 

 point. The system of organic rays is strictly a cytoplasmic system, 

 and yet the most important process in mitosis is the division of the 

 chromosomes. Rabl had both these factors of cell organization in 

 mind, but his own observations were directed most successfully to the 

 establishment of the polarity of the nucleus. In view of these facts it 

 is hard to see the basis for Heidenhain's low estimate of Rabl's work 

 (41, pp. 698-702). 



Against the sweeping contentions of Boveri and Heidenhain there 

 is, none the less, an abundance of evidence to be found in the work of 

 some of the best students of the animal cell. Meves (63, p. 47) holds 

 that the connections between centrosomes and chromosomes by the 

 so-called mantle fibers in the spermatocytes of the salamander are visi- 

 ble much earlier in the prophases than Hermann (43) admits, and 

 accepts the conception that the mantle fibers arise directly from the 

 linin network. Kostanecki (53, 54), developing still further the con- 

 ception of a system of organic rays advanced by Heidenhain, holds that 

 all the fibrous elements of the karyokinetic figure are reproduced by 

 longitudinal division during mitosis, and regards each ray and spindle 

 fiber as a permanent cell structure. 



Conklin, who has studied the mutual relations of the cell structures 

 more fully than any other investigator and has shown the relative posi- 

 tions of nucleus, centrosome, sphere substance, etc., through the resting- 

 stages as well as in karyokinesis, holds (17, p. 106) that it is evident 

 that some kind of connection exists at all stages of the cell cycle between 

 the centrosome and the nucleus. He further states that "whether 

 this connection during the rest is in the form of fibers (possibly a per- 

 sistence of those which previously connected centrosome and chromo- 

 somes) or is the expression of some other mechanical action or of 

 chemotropic attraction, does not appear from my studies." Conklin 



