276 PROCEEDINGS OF THE AMERICAN ACADEMY. 



region where a zygosporic line was commencing to form. lu the cou- 

 jugatious (a) and (c) the zygophores met terminally; in (b) a zygo- 

 plioric branch arose from the (— ) mycelium to come in contact with the 

 side of a (+) zygophore which had grown beyond it; and in (d) and (e), 

 the early stages for which are not represented, the ends of the zygo- 

 phores, when they had grown slightly past each other, were attracted 

 laterally so as to meet somewhat behind their terminations. Two min- 

 utes before contact occurred at (d) and while the hyphue were separated 

 by a distance equal to about a third of their width, very slight protrusions 

 were observed on the sides mutually facing (Figure 31), seemingly as 

 if the forces which were drawing the filaments laterally had effected a 

 bulging of the delicate walls at the growing points. Figure 33 is a 

 horizontal view taken from a young zygosporic line, and demonstrates 

 even more strikingly than the vertical view already described, the 

 mutual attraction which the zygophores exert. 



The stimulus for the formation of the progametes is thus very evi- 

 dently the contact between the sexually opposite zygophores, but what 

 is the nature of the stimuli which result in the formation of zygophores 

 and subsequently cause them to approach one another is by no means 

 clear. It has been demonstrated that zygospores would be produced in 

 the case of Rhizopus (p. 270) when only the aerial hyphae of opposite 

 strains were allowed to grow in contact, and therefore that the stimuli to 

 the origin or direction of growth of the conjugative hyphae were com- 

 municated solely through the air. From the habit of the species a similar 

 experiment with M. JIucedo is much more difficult, and the results which 

 have as yet been obtained are inconclusive. 



External Conditions. 



No systematic attempt has been made to determine the effects of vary- 

 ing external conditions on the formation of zygospores in this species. 

 In the numerous cultures which have been made, however, it has been 

 observed in general that, as with the other forms investigated, unfavor- 

 able conditions check the production of zygospores before that of spo- 

 rangia. Thus in cell and slide cultures, perhaps because of the thinness 

 of the nutrient layer, and in cultures affected with bacteria, zygospores 

 are not copiously developed. By growing the strains opposed at a tem- 

 perature of 2G°-28° C, it is also possible to prevent conjugation entirely. 

 Moreover, if potato agar, upon which zygospores readily form, be acidu- 

 lated with orange juice and used as the substratum, no conjugations can 

 be obtained. In general it may be said that in this, as in other cases, 



