248 REDUCTJOX OF THE CHROMOSOMES 



but half the usual number both of chromosomes and of ids. This mode 

 of tetrad-formation has been most clearly demonstrated in insects 

 and copepods. and an equivalent ])rocess occurs also in mollusks, 

 annelids, turbellarians, and some other animals, as described beyond. 

 In the second type, illustrated especially by Ascaris, the tetrad is 

 apparently formed by t:^>o longitudinal divisions of each primary 

 chromatin-rod, and no reducing division occurs. If, therefore, we 

 adopt the same terminology as before, we have first ab and cd. then 



it _ 12/, and hnall V — !— - — — , bv two longitudinal divisions. In 

 ab id ' ab ! ab cd ^ cd 



this case, according to Brauer's careful studies, each chromatin-granule 

 (** id") divides at each longitudinal division of the primary rod. The 

 four chromosomes of the tetrad are therefore exactly equivalent, being 

 derived from the same region of the spireme-thread, and containing 

 the undiminished number of " ids " (Fig. 121, II). 



The contradiction may be stated in a different way. In the first 

 type of tetrad formation, the number both of granules and of chro- 

 mosomes is first doubled {i.e. in the assumed case, through the forma- 

 tion of two tetrads, each consisting of four chromosomes, or eight in 

 all), and then reduced to half that number by the two successive matu- 

 ration-divisions. In the second type, on the other hand, the number 

 of chromosomes is likewise doubled, but that of the granules is quad- 

 rupled, so that, although in both types the two maturation-divisions 

 reduce the number of elirouibsonies to one-half, only in the first type 

 do they reduce the number of granules or **ids," as Weismann's 

 hvpothesis demands. We must therefore distinguish sharply between 

 the reduction of the chromosomes and that of the *'ids." The former 

 is primarilv effected b\' the segmentation of the jDrimary s])ireme- 

 thread, or the resolution of the nuclear reticulum, into one-half the 

 usual number of segments (?>. the "pseudo-reduction" of Riickert); 

 and here the real secret of the reduction of the chromosomes lies. The 

 reduction of the "ids," if they have any real existence, is a distinct, 

 and as yet unsolved, question. 



2. Detailed Evidence 



We may now consider some of the phenomena in detail, though the 

 limits of this work will only allow the consideration of a few typical 

 cases. 



{a) Tetrad for)nation ivith one Longitudinal and one Trajisverse 

 Division. — In many of the cases of this type the tetrads arise from 

 ring-shaped bodies which are analogous to the ring-shaped chromo- 

 somes occurring in heterotypical mitosis (p. %6). First observed by 

 Henking('9i) in /^j';r//^rf7;7i-, tetrad-origin of this type has since been 

 found in other insects by Vom Rath, Toyama, Paulmier, and others, 



