274 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Cell and slide cultures, which are so advantageous for other forms, are 

 unfavorable to the production of zygospores. In the majority of the 

 observations made, the (+) and (— ) strains were contrasted on nutrient 

 ao-ar in Petri dishes, and their growth was examined with the Zeiss A 

 objective. The thin sheets of glass which have been used as covers need 

 to be frequently removed and cleared from the moisture which condenses 

 on their under side from the evaporation of the substratum. It is for 

 this reason perhaps that the best results' have been obtained during damp 

 weather, when the drying effect of such an exposure to the air of the 

 room has been lessened. 



In the species under consideration, no difference in the growth of the 

 mycelia of the two strains has been observed which can be used to 

 distinguish them. From both arise scattered more or less branched 

 aerial filaments which from the very first are slender and apparently re- 

 main sterile, being readily distinguished from the comparatively stout 

 hvphae destined to become sporangiophores, by their more delicate habit, 

 and by the fact that they are not heliotropic. An exaggerated produc- 

 tion of these filaments which may occur on certain nutrients often gives 

 a felted appearance to the mycelial growtli. Erect filaments of this na- 

 ture occur throughout the whole mycelial area, but where the mycelia of 

 the opposite strains come together hyphae are produced which, while 

 they are intermediate in size between the sterile filaments just men- 

 tioned and the stout young sporangiophores, agree with the former in 

 not being heliotropic. 



Direct observation of these hyphae seems to indicate that a mutual 

 attraction, which may be termed zygotactic, is exercised between the 

 zygophoric hyphae belonging to opposite mycelia, and they may be seen 

 gradually to approach each other, but in the minority of cases is their 

 contact exactly terminal (Plate II, Figure 30c). In some instances the 

 point of contact may be slightly back of their tips (Figure 32d), or one 

 zyijophore may be laterally met by the end of the other (Figures 30b 

 and 33), and in other cases it may even happen that both zygophores 

 touch laterally. At the point where the opposite zygophores come in 

 contact, a swollen progamete is rapidly developed from each hypha, 

 and, according to the position in which the zygophores have met, appears 

 either lateral or terminal. The progametes, however, in all cases are, 

 from the very first, mutually adherent, and by their enlargement push 

 apart the zygophoric hyphae from which they have originated. These 

 latter do not seem to undergo any alteration other than a slight curva- 

 ture in some instances, occasioned by the lengthening of the progametes, 



