750 INHERITANCE 



over between dioecious races having sex factors of different strengths. 



In this article of 1938, Moewus discusses the difficulties arising in 

 his data as to crossing over. In all the data presented by him up to this 

 time, crossing over was reported as occurring in accordance with the two- 

 strand schema. Any zygote that gave any crossover combination gave 

 such combinations exclusively. Moewus now concludes that this is not 

 the normal state of affairs; he characterizes it, indeed, as pathological. 

 Under certain conditions, he reports, crossing over occurs in the normal 

 manner, according to the four-strand schema. He promises a future ac- 

 count of detailed investigations showing this. 



Certainly there is great need for clearing up the confused situation as 

 to crossing over in these organisms. In hundreds of detailed earlier re- 

 ports, Moewus has given data that are consistent only with two-strand 

 crossing over. A further serious criticism, based on other grounds, has 

 been made as to the accuracy of Moewus's data on crossing over, by 

 Philip and Haldane (1939). 



Aside from this difficulty, the work of Moewus has placed the genetics 

 of the Protozoa on a new footing. It has brought the phenomena of 

 inheritance in these organisms into the same system that is manifested in 

 the Mendelian inheritance of higher organisms. It has brought to light 

 in the flagellate Protozoa instances of most of the phenomena in such 

 inheritance, as before known in multicellular organisms. 



BiPARENTAL INHERITANCE IN DIPLOIDS: CiLIATA 



The genetics of diploids is necessarily more complex than that of 

 haploids. The individuals have during active life two sets of chromo- 

 somes instead of one. In consequence, in the sexual process two indi- 

 viduals do not normally unite completely, to form a diploid zygote, as in 

 the copulation of the flagellates. In the diploid ciliates, two individuals 

 merely come into intimate contact and exchange pronuclei that contain 

 each a haploid set of chromosomes, a process known as conjugation. The 

 two then separate, each carrying two haploid sets of chromosomes, one 

 from each of the conjugant individuals. Each then multiplies vegetatively, 

 forming clones, all members of any clone having the same diploid com- 

 bination of chromosomes. Conjugation thus is like fertilization in higher 

 organisms, in that it produces new diploid combinations of chromosomes. 



Important for the understanding of inheritance in conjugation are the 

 following: 



