KARYOKINESIS. 



11 



arms which lie in the spindle axes lengthen, while the transverse arms shorten, the 

 hole disappearing ; in this way chromosomes are formed with three enlargements (C, 

 4 and 5) similar in all respects to those described above (B, 4 and 5). The resemblance 

 of these two forms is so close that it is difficult to explain the differences in their 

 mode of origin. It is possible that forms, such as those shown in B, 4 and 5, are 

 really crosses with short transverse arms, the tips of all four arms being bent toward 

 each other until they nearly- or quite meet. 



The striking differences in the shapes of the chromosomes of the prophase is 

 continued into the metaphase where at least three distinct types ma}- be recognized 

 as shown in text fig. I, lilies A, B and C. In the late anaphase, however, all come 

 back to a cubical or tetrafoil condition ; a hole is usually present throusih the 

 middle of these as in the prophase. 



B 



OO ) t 



Prophase 



6 7 8 9 



I 



Metaphase 



10 11 



I « 



t o 

 i o 



Anaphase 



Fig. I. — Cliromosomes of the First Maturation Division of Ci-epidula. 



Various authors have called attention to the variet\- in the form of the chro- 

 mosomes of the first maturation division (v. Klincktnvstrom '96, Van der Stricht, 

 '98, Foot '98, Lillie '98, Griffin '99). Foot and Lillie figure 8-part chromosomes in 

 the metaphase of tlie first maturation of Alloloboplwra and Unio entirely similar 

 to those in Crcpidula, and Lillie shows these cliromosomes split longitudinally, as 

 they must be, if formed from crosses as shown in text fig. I, line C. 



It is difficult to say whether tliese differences in the shapes of chromosomes 

 mean iiitich or not. On the one liand it is possible that all tlic chromosomes of a 



