248 REDUCTION 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 process 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 tzvo 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 



ab cd 



fi^-£^, and finally 4 

 ab cd ab 



— , by two longitudinal divisions. In 

 cd 



ab 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 chromosomes to one-half, only in the first type 

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

 hypothesis demands. We must therefore distinguish sharply between 

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

 is primarily effected by the segmentation of the primary spireme- 

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

 usual number of segments (z.^. the "pseudo-reduction" of Riickert); 

 and liere 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. 



(«) Tetradformation with one Longitudinal and one Transverse 

 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. 86). First observed by 

 HenkingCQi) in Pyrrhocoris, tetrad-origin of this type has since been 

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



