36 NATIONAL TRENDS IN BIOLOGY 



animal is the product of the activity of all the genes to- 

 gether, while heredity is the result of the shuffling of these 

 genes in each generation. 



Quoting T. H. Morgan, who is the outstanding Ameri- 

 can in this field of work : 



"In several plants and animals we can refer these genes 

 to particular chromosomes, and in one animal at least, 

 about four hundred genes have been placed. On the 

 assumption of their relative positions with respect to one 

 another, we can predict what the numerical results will 

 be in inheritance for some four hundred different charac- 

 ters. The theory justifies itself in that it allows one to 

 predict the outcome in terms of numbers for all these 

 four hundred characters whose genes have been located. 



"The genes divide when the chromosomes divide, and 

 collectively their division is what we see when each 

 chromosomal rod splits throughout its length. It is the 

 genes that come together with extraordinary precision, 

 which implies probably that we are dealing with events 

 of a molecular order. We can go no further until physics 

 has furnished us with a key to unlock these extraordi- 

 nary events."^ 



Boveri's work on the mechanics of cytology must not 

 be left unmentioned at this point. 



So far we have discussed the work on cytology, genetics, 

 and heredity, embryology and regeneration, and physiol- 

 ogy with its subheadings of physics and chemistry. One 

 should not forget the exceptionally interesting and valu- 

 able work on tissue culture outside the body of the indi- 



