468 Keene . — Cytological Studies of the 
generation. This is purely hypothetical, however, and remains as a problem 
for further study. 
Investigations with Rhizopus nigricans and Zygorynchus moelleri are 
well under way. While these are by no means complete, yet I am con- 
vinced that the essential features are identical with those of Sporodinia. 
Zygorynchus has fewer nuclei, but the zygospores are so small that new 
difficulties are to be encountered. The nuclei in Rhizopus are somewhat 
larger than in Sporodinia , and the oil plastids appear at about the same 
stage of development. A single large plastid is characteristic of the mature 
zygospores of both Rhizopus and Zygorynchus. 
Summary. 
1. In the early stages of copulation of the sexual branches, there is 
no difference morphologically between the two branches. Later, there 
is a characteristic retraction of the protoplasm of one of the gametangia. 
This may or may not be of sexual significance. 
2. The nuclei of the gametangia are small and show the same structure, 
size, and staining reaction as those of the mycelium. 
3. Vacuoles and cleavage furrows are formed. A new membrane is 
formed by ingrowth from the lateral walls. A central thickened portion is 
formed. 
4. The wall between the gametangia is dissolved, and the gametes fuse, 
the protoplasm of one flowing into that of the other. 
5. Nuclear fusions occur progressively as the protoplasmic masses mix. 
6. Two sizes of nuclei result : 
(a) Large nuclei, the results of fusions. 
(h) Small nuclei which fail to fuse. 
7. The formation of the second wall of the zygospore then takes 
place, and the unfused nuclei, as well as those of the suspensors, begin to 
show degeneration. 
8. Mucorine crystals are present through all the earlier stages. 
9. There next appear globular cytoplasmic structures, with which, from 
their first appearance, oil is associated. They fuse irregularly, and produce 
ultimately one or two large bodies which are saturated with oil, as determined 
by the osmic acid test and the solubility in chloroform. 
10. These large cytoplasmic bodies are undoubtedly related to the 
elaioplasts described in many plants, and have for their function the 
elaboration of an oil for reserve food. 
11. In the mature zygospore there are numerous characteristic nuclei 
formed by the fusion of the two nuclei. The protoplasm is reduced to a thin 
parietal layer surrounding the oil plastid. This state of affairs persists 
until germination. 
