REDUCTION WITHOUT TETRAD-FORMATION 



259 



who have studied the vertebrates find two longitudinal divisions ; 

 while opinion regarding the plants is still divided. 



{a) Animals. — In the gephyrean TJialasscma and the mollusk 

 Zirp/icea {¥\gs. 128-130) Griffin ('99) finds that the rings, arising as 

 described above, place themselves in the equator of the spindle with 

 the longitudinal division in the equatorial plane. They are then 

 drawn out toward the spindle-poles from the middle point, first 

 assuming the form of a double cross, then of elongated elUpses, and 

 finally break into two daughter-U's or -Vs. The first division is 

 therefore longitudinal. During the late anaphase the V's break at 

 the apex, the two limbs come close together, so as to give the decep- 



A 



4 



B 



Fig. 128. — Diagrams of reduction in the types represented by Thalassema (A) and Saia- 

 mandra (B). In both the first division is heterotypical. The second division (6) is transverse in 

 the first and longitudinal in the second. 



tive appearance of a longitudinal split, and are separated by the 

 second division (following immediately upon the first without inter- 

 vening resting stage). The latter is therefore a transverse division 

 (Fig. 130). An essentially similar result, though less completely 

 worked out, is independently reached by Bolles Lee ('97) in Helix ; 

 by Klinckowstrom ('97) in the turbellarian ProstJiecercBiis ; and by 

 Francotte ('97) and Van der Stricht ('98, i) in Thysansoon. Klinckow- 

 strom shows that there is much variation in the way in w-hich the 

 rings open out and break apart, though the result is the same in all. 

 In case of the vertebrates, Flemming ('87) long since described 

 and figured typical tetrads in the salamander, but regarded them as 

 "anomalies." Vom Rath's later conclusion ('93, '95) that they are 



