ROBERTSON: THE CHROMOSOME COMPLEX. 283 



SYRBULA ADMIRABILIS. 



This usually gives an enlargement at this point, or, if far 

 enough along, a transverse extension. The point at which the 

 distal ends unite, when they do so in forming a ring, is often 

 marked by a constriction. The signs XX mark the points at 

 which the spindle fibers will be attached, and also where the 

 future cross or reduction division in the second spermatocytes 

 will take place. The degree to which the chromatids have 

 moved along the transverse axis determines the shape of 

 the tetrad. In this way we account for the various crosses 

 that are found. If the chromatids have moved out very far, 

 the long axis of the cross may be in the transverse direc- 

 tion. This condition may be seen in the long drawn-out chro- 

 mosomes of the lateral views (figs. 34 and 35). Exactly the 

 same phenomena may occur in the case of the ring forms. The 

 only difference with these is that the ends of the longitudinal 

 or primary axis of the cross are turned over toward each other 

 and united end to end, thus forming a ring. As the arms of 

 the transverse axis elongate or pull out, the ring (arms of the 

 longitudinal axis) keeps getting smaller (fig. 29) until there 

 is produced the same result as with the cross forms. One 

 of these rings acting in this manner is seen in figure 29. 

 From the figures in 29 we are led to believe that the greater 

 part of this gliding of the chromatids from the longitudinal 

 to the transverse axis is done while the chromosomes are still 

 in the prophase condition, before they enter the metaphase 

 and have had a chance to come under the influence of the 

 spindle fibers. This seems to indicate that chromosomes are 

 to a certain extent automatic in the division process. Lastly, 

 a word should be said in regard to the puzzling appearance 

 of some of the strictly cross forms. The angles that the limbs 

 of the cross may have with each other, depending upon the 

 position of the tetrad in the cell, often give to such chromo- 

 somes the appearance of K's, Y's, etc., but with careful ob- 

 servation it may easily be seen that they are of the cross type. 

 The figures themselves (fig. 29) furnish a better explanation 

 of this than is possible by the use of words. 



It is from prophase chromosomes such as we have been de- 

 scribing that the metaphase chromosomes of the first sper- 

 matocyte have been derived. The only difl^erence is that the 

 latter are much more condensed, even so much so that the 

 longitudinal split is no longer visible. But the same four 



