26o REDUCTION OF THE CHROMOSOMES 



normal tetrads has not been sustained by the still more recent work 

 of Meves ('96), whose caretul studies, together with those of Moore, 

 Lenhossek, and others, thus far give no evidence of tetrad-formation, 

 and seem opposed to the occurrence of reducing divisions in the 

 vertebrates. Mexes's work in the main confirms the earlier results 

 of Flemming, except that he shows that, as in so many other animals, 

 only two generations of spermatocytes exist. At the first division 

 the nuclear reticulum resolves itself into twelve (the reduced num- 

 ber) segments, which split lengthwise, the halves remaining united to 

 form elongated rings (Figs. 2j, ij). These do not, however, con- 



• • 





*•• • 





^ 



B 





• • • • • 



mh 



• ♦ . . . . • 





D 



Fig. 129. — Maturation and fertilization in an annelid (armed geplijTean) Thalassema. 

 [Griffin.] 



A. A few moments after entrance of the spermatozoon, showing accessory asters; tetrads 

 forming. B. Early prophase of first polar mitosis with centrosomes. C. In-pushing of nuclear 

 wall. D. Central spindle established; elimination of nucleolus and nuclear reticulum. E. Slightly 

 later stage viewed from above. F. First polar spindle established, cross-shaped tetrads, crossing 

 of astral rays; sperm-head at j. 



dense into tetrads, but break apart during the first division at the 

 points corresponding with the ends of the united halves. The first 

 division is therefore an equation-division. As the V-shaped halves sep- 

 arate they again split lengthwise (Fig. 131), each of the secondary sper- 

 matocytes receiving twelve double V's or dyads. In the telophases 

 and ensuing resting stage, however, all traces of this splitting are 

 lost, the nuclei partially returning to the resting stage, but retaining 

 traces of a spireme-like arrangement (Fig. 131). In the second 

 division twelve double V's reappear, showing a longitudinal division 

 which Flemming and Meves believe to be directly related to that 



