348 



Prof. E. B. Wilson. 



elaborated in a remarkable manner by Morgan and by Stnrtevant. This 

 explanation is as follows : — 



It has long been known that subsequent to the process of conjugation — 

 sometimes, as now seems probable, during this process — the two chromosomes 

 of each pair, while still in the form of long threads, often twist about each 

 other, thus producing the so-called " strepsinema " stage. Janssens concluded 

 from a careful study of the facts in the Amphibia that the double spirals thus 

 formed may in some cases come into contact and fuse at certain points where 

 the threads cross, and then may be separated again by a straight longitudinal 

 split through the points of fusion. Such a process would lead to an exchange 

 of certain regions between the two threads. Janssens named this the 

 " chiasmatype," and briefly called attention to the possible bearing of such a 

 process on the Mendelian phenomena. Morgan afterwards very ingeniously 

 developed this thought as follows : The " determining factors " or " gens " of 

 unit-character are assumed to be aligned, as Eoux long since suggested, in 

 linear series in the chroniatiu-threads and in a definite order. In the process 

 of conjugation corresponding or allelomorphic gens (large and small letters 

 in the diagrams) are assumed to lie opposite one another in the two threads, 

 as is shown in diagram in figs. 4 and 5. If in a certain proportion of cases 



Diploid Group 

 Complete lieterozygote (o) 



V 



w 



Diploid Group 

 Partial Heterozygote (9) 



II 



JEE If 



M. 



M 



K R 



V V 



II II WK 



Fig. 5. — Diagram illustrating heterozygous conditions. Homologous or allelomorphic 

 factors {e.g., C and c) occupy corresponding loci or levels in homologous chromo- 

 somes. The condition at the left is heterozygous for all factors ; at the right, 

 heterozygous for some (Aa, gG, etc.), and homozygous for others (BB, ee, etc.). 



the two threads (linear series of gens) twist together, unite, and separate in 

 the manner described by Janssens, the result will be an exchange between 



