THE (;i:R.M PLASM TIIKORY 307 



smaller elements. These diU'ereiillv .sized (1\ .ids an- 

 considered by Carotliors as "distinct pliy.siolo^n'cal 

 individuals, representing^ resj)ecli\ <*ly I he j)at('riial 

 and maternal contribution to the formation of some 

 character or characters; and, as each can he iden- 

 tified, they furnish an excellent means of tracing tlie 

 process of segregation and recombination '' (p. 499). 

 It was at first assumed that eacli of tlie pairs of 

 chromosomes which unite in synapsis was respon.sible 

 for a single adult ^ 



character, birt ^ U 



the number of d A /^/"^^i^O*^ 



Mendelian char- ^rp 0% Q P9 VAAj^ 



acters is known ^ 



, I , • Fig. 83. — Arjjftia simplex. Chromosomes ol 



LO oe greater m ^^.g^ spermatocyte, o = accessory chromo- 

 Certain cases than '^o"'^ • ^ = unequal dyad. {Frum Caruthers, 



the number ot 



chromosomes. Fortunately, it has been f()nn<l that 

 the characters, instead of unch'rgoing iuih'peiKh'ut as- 

 sortment, mav become hnked so that certain of them 

 almost always occur together in the oil*sj)ring. The 

 relation of these facts to the constitution of the 

 chromosomes mav best be il hist rated bv reference to 

 the studies of Morgan and his stu(h'nts on the fruit - 

 fly, DrosopJiila. Over one hundred nuitants of tliis 

 species have been discovered by these in\estigators. 

 So far as studied, the characters of lliese Hies seem to 

 form three groups. "The characters in t he first gronj) 

 show sex-linked inheritance. They follow the sex- 

 chromosomes. The second group is less extensive. 

 Since the characters in this group arc linked to each 



