98 KANSAS UNIVERSITY QUARTERLY. 



the approach of warm weather. Late stages of the spireme show un- 

 mistakable evidences of a longitudinal cleft. 



3. The segments of the chromatic thread offer clear and convinc- 

 ing evidence that the resulting chromosomes are quadrivalent struc- 

 tures. They may exist in the form of rods, double -V figures, or rings, 

 but all are modifications of one tyjje. In this the segment is split 

 longitudinally, and again in the center at right angles to the first cleft. 

 The members of this compound unit are able to move upon each other 

 so as to produce cross-figures with arms of various proportionate 

 lengths. 



4. A concentration of the loose elements of the prophase produces 

 the mature chromosomes of the first spermatocyte metaphase. This, 

 from the method of its production, is known to be quadrivalent, al- 

 though planes of division are usually indistinguishable. In the equa- 

 torial plate, the chromosomes arrange themselves with their longer 

 axis at right angles to the spindle. Fibers attach to the chromosomes 

 at their middle point, which corresj^onds to the place where the cross- 

 division was previously noted. 



5. By contraction of the fibers, the elements of the tetrad are re- 

 moved from each other in pairs. Separation takes place along the 

 plane corresponding to the original longitudinal cleft, but without 

 divergence of the chromatids. In this way the chromosome in the 

 final stage of separation resembles the one just beginning to divide, 

 except that its longer axis is parallel to the spindle axis. Movements 

 of all the chromosomes in a nucleus are not coincident, so that the 

 various stages may be noted in the same cell. 



6. The balance of forces inherent in the tetrad chromosomes ap- 

 pears to be destroyed when the diads separate, for the chromatids 

 composing these bipartite elements immediately diverge when they 

 lose connection with their sister pairs. They thus travel to the poles 

 in the form of a V-shaped structure more or less closely united at the 

 apex. During the telophase they become closely aggregated but never 

 lose their identity. There is, accordingly, no intermediate rest stage. 



7. The second spermatocyte division figure is established almost 

 as soon as the daughter cells are separated by the growth of an inter- 

 posing wall. In it, the spindle is small, and the chromosomes are 

 arranged on its periphery in such a way that the chromatids of each 

 pair are superimposed in the plane of the spindle axis. Fibers attach 

 at the same place they did in the first spermatocyte and easily sepa- 

 rate the loosely joined chromatids. These move apart to the poles 

 and coincidentally the cell and spindle increase in length. Since the 

 fibers of the first spermatocyte figure were attached to the chromatids 

 at a point marking the plane of cross cleavage, the second spermato- 

 cyte mitosis which separates them at this point is a cross-division. 



