44 THE THEORY OF THE GENE 



of the eggs of Planaria (Fig. 28) are also quite convinc- 

 ing. Some at least of these figures give the impression 

 that as the threads are coming together they overlap one 

 or more times, but this impression is not sufficient to 

 show that they do more than lie across each other as seen 

 from certain levels. It does not follow moreover that they 

 will interchange where they overlap. While it must be 

 admitted, then, that the cytological evidence of crossing- 

 over has not been demonstrated, and from the nature of 

 the conditions it will be extremely difficult to actually 

 prove; nevertheless, it has been shown in a number of 

 cases that the chromosomes are brought into a position 

 where such an interchange might readily be supposed to 

 take place. 



The cytologist, then, has given us an account of the 

 chromosomes that fulfills to a degree the requirements of 

 genetics. When we recall the fact that much of the evi- 

 dence was obtained prior to the rediscovery of Mendel's 

 paper, and that none of the work has been done with a 

 genetic bias, but quite independently of what the students 

 of heredity were doing, it does not seem probable that 

 these relations are mere coincidences, but rather that 

 students of the cell have discovered many of the essential 

 parts of the mechanism by which the hereditary elements 

 are sorted out according to Mendel's two laws and are 

 interchanged in an orderly way between members of the 

 same pair of chromosomes. 



