GENETICS — MORGAN 



351 



repulsion of the members of individual chromosomes, but we have 

 no knowledge of the kind of physical processes involved. The terms 

 attraction and repulsion are purely descriptive, and mean no more 

 at present than that like chromosomes come together and later 

 separate. 



In earlier times, when the constitution of the chromosomes was not 

 known, it was supposed that the chromosomes come together at ran- 

 dom in pairs. There was the implication that any two chromosomes 

 may mate. The comparison with conjugation of male and female 

 protozoon, or egg and sperm-cell, was obvious, and since in all diploid 

 cells one member of each pair of chromosomes has come from the 

 father and one from the mother, it must have seemed that somehow 

 maleness and femaleness are involved in the conjugation of the 



Scjie |4 imm >-' 



SALIVARY GLAND 



Secreier; cells 

 Ctilt McrmMU {iOfi) 

 ChrMMssntcs tfi»rnieu>(490^) 

 tni tSe« s«9lK) of citil} 

 test) iottrnel etnielvrs 



BRAIN 



Ctiiglien c«IU. 

 C<IU Icffn (12/t) 

 CSvmMMDin lerstr « tt) 

 tli«eiRg Ructasli.conafriclicm, 

 *IM lifit «n4 dark region 



GONAD (Testis) 



Ctrm-tncli celU(jom«l) 

 C«lb small (7/i) 

 Chfcmowmei imall [Xiu) 

 but iharp in outline (ntf 

 sesy te ccunt 



FiGDRE 5. — Diagram of larva of Drosophila tnelanogaster, showing the gonad, brain, and 



one of the salivary glands. 



chromosomes also. But today we have abundant evidence to prove 

 that this idea is entirely erroneous, since there are cases where both 

 chromosomes that conjugate have come from the female, and even 

 where both have been sister strands of the same chromosome. 



Recent genetic analysis shows not only that the conjugating chro- 

 mosomes are like chromosomes, i. e., chains of the same genes, but 

 also that very exact processes are involved. The genes come together, 

 point for point, unless some physical obstacle prevents. The last 

 few years have furnished some beautiful illustrations showing that 

 it is genes rather than whole chromosomes that come to lie side by 

 side when the chromosomes come together. For example, occasion- 

 ally a chromosome may have a piece broken off (fig. 2, above) 

 which becomes attached to another chromosome. A new linkage 

 group is thus established. When conjugation takes place this piece 



