176 



CYTOLOGY 



CHAP. 



intimate nature of the syndesis proper, has obviously the function of 

 bringing the homologous chromosomes in pairs on to the meiotic spindle. 

 This being effectively achieved by this late prophase pairing, we are 

 compelled to look for another function for syndesis in these cases, and 

 this function, as indicated above, we believe to be the exchange of 

 hereditarj^' factors. 



In what way may we conceive that this hypothetical interchange of 

 factors is effected ? Two possibiHties have been suggested — the first 

 is that which naturally presents itself, namely, that while the chromo- 

 somes are longitudinally apposed to one another in parasyndesis, an 

 exchange of chromatin units takes place analogous to the exchange of 

 nuclei between two conjugating Infusoria. The other suggestion was first 



made by Janssens (1909) (chiasma- 

 typie) on purely cytological grounds. 

 Since the homologous chromosomes 

 separating after syndesis are often 

 spirally twisted round one another 

 (strepsitene stage), he suggested that 

 fusion may occur at the points of 

 junction, and that when these break 

 apart at final separation the chromo- 

 somes may be composed of alternate 

 segments of the original chromosomes 

 (Fig. 78). 



A view similar to Janssens' has 



B 



Fig. 78. 



Diagram illustrating the hypothesis of " chias- 



matypie" or "crossing over." A, a pair of bcCU SUppOrtcd by Morgan, CtC, aS 

 chromosomes before syndesis ; B, diplotene . i • i r t • r 



(= strepsitene) stage; C. the chromosomes fully forming a CytolOglCal fOUndatlOU for 

 separated after syndesis. . • ,i rs 



certam breedmg results with Droso- 

 phila and other forms. It is known as the " crossing over " hypothesis, 

 and is supported by the following experimental evidence. 



A number of cases are now known in which characters which can 

 be separated in crossing, nevertheless show a preference to remain in 

 the combinations in which they were present in the parents, rather than 

 to become rearranged in different combinations. Thus if two forms, 

 differing as regards two characters, are crossed, say ^5 x ab, the F-^ hybrid 

 will of course form four classes of gametes AB, Ab, aB, and ab. In most 

 cases these classes are formed in equal numbers, but in some cases the 

 gametes representing the parental combinations are found in excess of 

 the others. In the cross AB xab we have an excess of the gametes AB 

 and ab, and a deficiency of the Ab and aB classes. On the other hand, 

 the cross Ab xaB gives an excess of Ab and aB and a deficiency oi AB 

 and ab. It must not be supposed that this is a mere exception to the 



