JUNE, 

 1897. 



CHROMOSOME-REDUCTION AND WEI S MANN. 407 



stage in the maturation of the sexual cells, elsewhere termed by me 

 the " synaptic rest " because this rest is universally followed by the 

 disappearance of the regular number of somatic chromosomes and 

 the evolution of half that number of chromatic rings (heterotype 

 chromosomes). The succeeding heterotype division is generally 

 followed by another, in which the same reduced number of chromo- 

 somes is retained. Now Hacker, together with Vom Rath (3) and 

 Riickert (5), has maintained that this last division differs from the 

 heterotype and all other mitoses, in that here the chromosomes split 

 transversely, and not longitudinally as in every other observed case. 

 If this is so, and if we assume as Hacker does that the synaptic rest 

 results in the fusion of pairs of chromosomes end to end, then it will 

 be seen that if we represent, say four chromosomes, typical of the 



Figures 1-8. — Methods of Chromosome-reduction. 



somatic divisions of any species, by the letters a, b, c, d (fig. 2), the 

 effect of the synapsis will be to unite these chromosomes together in 

 pairs as in fig. 3. In the ensuing division these double chromosomes 

 (" plurivalent chromosomes " of Hacker) spUt longitudinally to form 

 the double V's (" tetrads " of Hacker) as represented in fig. 4. These 

 bodies subsequently divide, in the manner represented in fig. 5, to 

 form two new cells each with two compound chromosomes, as in 

 fig. 6 ; and according to Hacker, the final division is brought about 

 by these compound chromosomes becoming disunited from one another 

 and passing unsplit into each daughter-cell. It is thus possible to 

 imagine that in this way a distribution of the chromosomes may be 

 brought about exactly similar to that which Weismann originally 

 conceived, and that this final transverse splitting may be really what 

 Weismann's postulate requires. 



