ORIGIN OF THE TETRADS 



247 



A B C D 



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a 



I 



5 



6 



7 

 ^8 



a 



, a b 



chromosomes 



b 



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rm-mi 



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00 



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4>0 



rt' 



(7 



a 



usual number of pieces. Apparently, however, there are two radi- 

 cally different types of tetrad-formation as follows. 



In the first type the tetrad arises by one longitudinal and one trans- 

 verse division of each pj-iuiary c/ironmtin-rod, the latter effecting the 

 reduction demanded by 

 Weismann's hypothesis(Fig. 

 121, I). To give the usual 

 graphic representation, let 

 us, for the sake of discus- 

 sion, assume the somatic 

 number of chromosomes to 

 be four, designating the 

 spireme-thread as a b c d, 

 each letter representing a 

 chromosome, each of which 

 we may in turn assume to 

 consist of a series of four 

 granules or " ids " ( Fig. 121). 

 In ordinary mitosis the spi- 

 reme would segment into 

 a — b — c — d, which then 

 would divide lengthwise to 

 form pairs of identical sister 



_d 



a c d 

 To form the tetrad, on the 

 other hand, the spireme first 

 segments into two rods ab 

 and cd, each of which, in 

 view of its subsequent his- 

 tory, may be regarded as 

 bivalent, representing two 

 chromosomes united end to 

 end (Vom Rath, Ruckert, 



5 



6 



• 8 



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4» O 



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•-« 



IH> 



iVW 



8^48 



f0 



a 



si 08 «?• 



a 



ab cib 



ab cib 



*<f 



8 



Fig. 121. — Diagrams of tetrad-formation; I, wiili 



one transverse and one longitudinal division (copepod 



type) ; II, with two longitudinal divisions {Ascaris type). 



A-D, successive stages ; chromatin-granules num- 



Hacker). Each of these bered from i to 8. The two types diverge at C. In D 



divides once longitudinally, thegranulesof each constituent of the tetrad fuse to form 



. . ^ -^ ' a homogeneous sphere. 



giving the identical pairs or 



c 



dyads -, and once transversely, giving the tetrads — 



ab cd a 



d^ 

 d 



Inspection of Fig. 121, I, shows that through the second or transverse 

 division, each member of the tetrad receives only half the number of 

 ids contained in the original segment. This number, four, is the same 

 as that assumed for a single chromosome ; and, since each of the two 

 tetrads contributes one chromosome to the germ-cell, the latter receives 



