BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 273 



lines of zygospores which resulted. By testing these nine pairs of pure 

 cultures, it was then deterinined that the mycelium derived from either 

 member of these pairs would take part in the formation of zygospores 

 when grown in contact with the mycelium of a certain member of any 

 other pair. By contrasting all the cultures with one another, it was 

 further found possible to arrange them all in two series in such a 

 manner that those in the opposite sei'ies would produce a line of zygo- 

 spores when grown together, while those in the same series would 

 produce no such line. It may be added that since this time zygospores 

 of this species have been procured from a number of different sources 

 and when the strains producing them have been separated they have 

 always arranged themselves in one or the other of the two series to 

 which, as has already been explained in the Introduction, the terms 

 (+) and (— ) have been applied. 



The appearence of the zygosporic lines which result when the my- 

 celia of (+) and ( — ) strains of this species are allowed to grow in con- 

 tact is illustrated in the culture photographed (Plate IV, Figure 55), 

 where inoculations of the two opposite strains were made as indicated by 

 the (+) and ( — ) signs. Between the mycelia distinguished by unlike 

 signs a black line marks the position of zygospore formation. The com- 

 paratively meagre development of zygospores in the middle of this cul- 

 ture is caused by the less depth of nutrient at this place due to a 

 convexity in the bottom of the Petri dish used for the culture. 



The presumption that the formation of zygospores is a truly sexual 

 process, and tluit in the two strains of this speci'^s we have represented 

 the two sexes is substantiated by the results obtained by hybridization to 

 be discussed in a later section. The fact that in this and certain other 

 species the sexes are separated in different thalli led to the use of the 

 term heterothallic to distinguish them from the homothallic forms in 

 which it must be assumed that both sexes are associated in the same 

 thallus. 



Morpholocjy and Phtjsiologij of Conjugation. 



Mxicor Mucedo, from the fact that its separate strains have been long- 

 est under cultivation, and from the comparative ease with which its 

 growth can be directly observed, has furnished most of the informa- 

 tion which has been obtained in connection with a somewhat detailed 

 examination of the process of conjugation. Many points of interest, 

 however, remain untouched, a further examination of which has been 

 deferred for the present;. 



VOL. XL. — 18 



