Keene—Studies in Zygospore Formation. 1215 
Dicranophora , homothallic forms, the morphological differen¬ 
tiation is marked and constant. 
Summary 
1. There are no morphological differences to be found in the 
germinating spores of the plus and minus strains of Phycomyces 
nitens. They both germinate by means of one or two germ 
tubes which branch and produce, the vegetative mycelium. 
2. The vegetative mycelium of the minus strain grows some¬ 
what less vigorously than that of the plus strain and the spor¬ 
angia appear later. 
3. The sporangia of the plus and minus strains are similar 
in all respects. 
4. The two strains—the plus and the minus—are necessary 
in order to secure zygospores. 
5. Zygospores were secured on various kinds of media and 
their production does not seem to be in any way dependent 
upon the concentration of the medium. Humidity, however, 
appears to be an important secondary factor. 
6. Interlocking of lobes occurs when certain hyphal branches 
of the plus and minus strains come in contact. This appears 
to be the result of contact and not of chemical stimulation. 
7. The terminal portions of these branches elongate forming 
the progametes. There is no difference morphologically be¬ 
tween the two progametes. 
8. The progametes show an increase in the number of nuclei 
as they increase in size and round out from each other. This 
increase in the number of nuclei is undoubtedly due to nuclear 
divisions. 
9. Delimiting walls are formed which cut off the terminal 
portions, resorption of the contiguous walls occurs, the two 
gametes come in contact, and a mixing of the protoplasm takes 
place. 
