THE MECHANISM OF HEREDITY 43 



about each other (Fig. 28b), but this interpretation is 

 often open to suspicion. It is extremely difficult, in fact, to 

 determine when threads as delicate as these come into 

 contact with each other whether they pass above or below, 

 i.e., above at one node and below at the next, etc. The 

 difficulty is enormously enhanced by the coagulated con- 

 dition of the threads, and it is only in this condition that 

 they are stained for microscopic study. 





nf 





it 



b 



i4 





■^^- 



Fig. 28. 

 Conjugation of a pair of chromosomes of a planarian. In a, the 

 two thin threads are coming together; in b, there are indications, 

 at two levels, of crossing-over of the two united strands. (After 

 Gelei.) 



The preparations that most nearly approach a demon- 

 stration of the twisting of the leptotene threads are those 

 in which the conjugation begins at one end (or at both 

 ends of bent chromosomes) and progresses toward the 

 other end (or toward the middle of the bend). The sperm- 

 cells of Batrachoseps presents perhaps the most seduc- 

 tive preparations of this sort (Fig. 26), but the figures of 

 Tomopteris are almost or quite as good. The drawings 



