748 INHERITANCE 



Moewus) took place in accordance with the two-strand scheme; any 

 zygote that gave crossover combinations at all gave only crossover com- 

 binations. 



In Protosiphon an investigation was made on the determination of 

 sex by external conditions. Protosiphon includes dioecious races, in which 

 the determination of sex is genotypic, as described in earlier pages; 

 monoecious races, in which a single clone contains individuals of both 

 sexes, the determination of sex being mainly phenotypic; and certain 

 other races that agree in some respects with one type, in some respects 

 with the other. The phenotypic determination of sex is not here dealt 

 with. 



Of interest for the nature of inheritance are the races which partake 

 of the features of both types. Such is the race d, described by Moewus 

 ( 1935c) . At a certain stage in its life history, Protosiphon is a small club- 

 shaped "haplont," containing many haploid nuclei imbedded in a mass 

 of cytoplasm without cell walls. Such a haplont may be produced by a 

 zygote resulting from the union of two gametes. In the diploid zygote, 

 the reduction division occurs and the haploid nuclei multiply, giving rise 

 to the haplont. Or haplonts may be produced each from a single haploid 

 swarm cell, which comes to rest on the surface of a solid (as agar) and 

 produces many nuclei by division of its single nucleus. Haplonts pro- 

 duced in either of these ways give rise later to swarm cells (gametes). 

 The nuclei separate, each is surrounded by a small mass of cytoplasm and 

 each such cell transforms into a flagellate swarm cell. In the race d, as in 

 dioecious races, all the swarm cells produced by a single haplont are of 

 the same sex. 



But, in the race d, the sex of the swarm cells produced by the haplont 

 depends on the conditions under which the haplont was produced. 

 Haplonts grown on acid agar give rise only to plus swarm cells. Those 

 grown on alkaline agar yield only minus swarm cells. Of those grown 

 on neutral agar, some yield only plus swarm cells, others only minus 

 swarm cells. The sex of the swarm cells and of all their descendants by 

 vegetative fission is determined once for all by the conditions prevail- 

 ing in the development of the haplont from which they arise. This is 

 true not only for haplonts produced from single swarm cells, but also for 

 haplonts produced from zygotes. Since in the zygote the reduction divi- 

 sion occurs, yet all the four cells resulting from the maturation divisions 



