75 



may be recognized even though they may have entered into a 

 genetic union with those of another race or species. 



Meiosis is defined by Darlington as two divisions of the 

 nucleus accompanied by one division of its two-parted chromo- 

 somes. It is at meiosis that the final evidence of chromosome 

 pairing in hybrids and non-hybrids, in polysomes, and in poly- 

 ploids occurs. It is the time when the results of fertilization can 

 be evaluated. That meiosis takes the complicated course that it 

 does in nearly all sexual plants and animals, has one object: 

 "it provides the conditions for crossing over without which 

 recombinations of genes and secondary structural changes in the 

 chromosomes would be excluded. Meiosis has no virtue except 

 in hybridity. Gene changes on the one hand and proportion and 

 quantity changes on the other hand are essential agents in 

 evolution." 



The pendulum swings back and it has retreated two hundred 

 years in time. The discarded preformationist has been resur- 

 rected, he is now in our midst — modernized to be sure — yet a 

 dominating figure. The chromosomes retain their individuality, 

 in them alone are the units of heredity, the genes. In the nuclei 

 are the predetermined representations of the generations yet to 

 be. The geneticist and cytologist have mapped out the chromo- 

 somes assigning definite places to the hundreds and thousands 

 of genes responsible for form and function. And just as the pre- 

 formationist of the 17th and the 18th centuries saw in the egg 

 the preformed chick, so does the cytogeneticist see the charac- 

 ters of stem, leaves, flowers, roots, tissues, eyes, wings, color, 

 and their endless morphological and physiological attributes in 

 the chromosomes of the egg and sperm. 



The role of the cytologist — the student of the physiology of 

 the cell has become more and more circumscribed. The edifices 

 that he laboriously erected have fallen into disuse and are no 

 longer esteemed. He has been forced to retreat from position to 

 position, giving up ground all the time. The cytoplasm and its 

 inclusions have been abandoned. Taking inventory of his wares 

 the cytologist finds that all that he has left now, are chromo- 

 somes and spindle fibers. His transactions are with these two 

 commodities. Genetics and cytology have entered into an in- 

 dissoluble bond; they have erected a new preformationist doc- 

 trine. 



