1 86 REDUCTION OF THE CHROMOSOMES 



B. ORIGIN OF THE TETRADS 

 i. General SketcJi 



It is generally agreed that each tetrad arises by a double division of 

 a single primary chro matin-rod. Nearly all observers agree further 

 that the number of primary rods at their first appearance in the 

 germinal vesicle or in the spermatocyte-nucleus is one-half tJic usual 

 number of chromosomes, and that this numerical reduction is due to 

 the fact that the spireme-thread segments into one-half the usual num- 

 ber of pieces. The contradiction relates to the manner in which the 

 primary rod divides to form the tetrad. According to one account, 

 mainly based on the study of Ascaris by Boveri, Hertwig, and Brauer, 

 and supported in principle by the observations of Guignard and 

 Strasburger on the flowering plants, each tetrad arises by a double 

 longitudinal splitting of the primary chromatin-rod caused by the 

 division of each chromatin-granule into four parts. In this case the 

 four resulting bodies i.e. the four chromosomes of the tetrad - 

 must be exactly equivalent, since all are derived from the same 

 region of the spireme-thread and consist of equivalent groups of 

 ids or chromatin-granules (Fig. 102, A). No reducing division can 

 therefore occur in Weismann's sense. There is only a reduction in 

 the number of chromosomes, not a reduction in the number of qualities 

 represented by the chromatin-granules. This may be graphically 

 expressed as follows: 



If the original spireme-thread be represented by abed, normal 

 mitosis consists in its segmentation into the four chromosomes 



a b c d, which split lengthwise to form - -, -, - In matu- 



a o c a 



ration the thread segments into two portions, ab cd, each of which 

 then split into four equivalent portions, giving the equivalent tetrads, 



ab 

 thus, r 



ab 



and 



ab cd 



cd x 



}> or - 

 cd x 



y 



, since it is not known 



y 



ab 

 whether ab really is equal to a -f b. 



The second account, which finds its strongest support in the 

 observations of Riickert, Hacker, and vom Rath on the maturation of 

 arthropods, asserts that each tetrad arises by one longitudinal and one 

 transverse division of each primary chromatin-rod (\g. 102, B). Thus 

 the spireme abed segments as before into two segments ab and cd. 



These first divide longitudinally to form and -- and then trans- 



ab cd 



a\b 1 c 



vcrsely to torm and 



a\b c 



. Each tetrad therefore consists, not of 

 d 



