BLAKESLEE. — SEXUAL REPRODUCTIOX IN THE MUCORINEAE. 261 



throughout the series, and the cooperation of which is necessary for the 

 process of conjugation, indicates that it is sexual in nature, and that the 

 (+) and (— ) strains represent the two opposite sexes. 



Investigations of the ^'Harvard Strain y 



A brief history of tlie " Harvard strain " has been given under the 

 citations in Part I (p. 233), where it was mentioned that the strain, after 

 running for nearly ten years in the laboratories of a number of different 

 institutions, had ceased to produce zygospores. Since, as we now know, 

 this " strain " was merely a chance admixture of (+) and (— ) strains 

 which had been preserved in its zygosporic activity by transfers of por- 

 tions of the substratum containing tlieir spores, it is a matter of interest 

 to discover the effect on the sexual strains of this ten years association, 

 and to learn in what way the culture has lost its power of producing 

 zygospores. 



Although since 1901 zygospores have not been obtained in the labor- 

 atory from cultures of the " Harvard strain," the material has been 

 preserved on pieces of dry bread. From such an old culture several 

 transfers were made to stenders of flour paste which, by failing to produce 

 zygospores, showed that not more than one of the sexual strains was 

 present. This, by several tests with (+) strains, has been found to be 

 (— ) in character, and the same is true of the Harvard material carried 

 to Dartmouth College in 1901 by Prof. G. R. Lyman, which was kindly 

 sent to the writer for examination. In both these instances the zygo- 

 spores produced by contrasts with (+) strains were perfectly normal. 



Dr. Otis W. Caldwell of Charleston, 111., has kindly sent the writer 

 material which traces descent to the " Harvard strain " through the 

 laboratory of the University of Chicago, and writes that he has been 

 unable to obtain zygospores from it since 1902. A culture from this 

 source fails to produce zygospores when contrasted with (-f) strains. 

 With (— ) strains, however, although a reaction is evident, accompanied 

 by the formation of numerous progametes, and the characteristic yellowish 

 tinge appears in the zygosporic apparatus, no perfect zygospores are 

 formed. One of the progametes generally remains small, and fails to 

 distinguish its gamete while the other becomes abnormally swollen, often 

 discolored, and more or less tuberculate throughout with a constriction 

 where a septum for the separation of its gamete might be expected. 

 These imperfect conjugations are suggestive of the conditions associated 

 with hybridization, and are similar to what has been observed of one 



