BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 301 



phae. They may both arise close together, witli a proximal protoplasmic 

 connection between them ; and while one remains but little greater in 

 diameter than the mycelial filament from which it arises, the other is dis- 

 tinctly swollen (Plate I, Fiijare 10). Stimulated by the contact, a bulge 

 develops in the large zygophoric branch at its point of meeting with the 

 other (Figures 10 and 11). When the gamete is distinguished from the 

 slender branch, the bulge opposite is about equal in size, but in the case 

 figured a septum has not yet formed distinguishing it as a gamete. 

 Apparently a rapid increase in size takes place before this septum forms, 

 and by the dissolution of the walls in contact an open communication 

 is established between the two conjugating cells (Figure 12). At this 

 stage of development the condition is essentially similar to that figured 

 in Zygorhyiichus (Figure 7). In the species under discussion, however, 

 the smaller gamete forms a less proportionate part of the zygote than 

 does the corresponding gamete in Zygorhynchus. The zygospore is 

 developed almost entirely within the stretched and finally cutiuized 

 walls of the larger gamete, and a small beak which is generally present 

 on the zygospore is the cutinized remnant of the wall of the smaller 

 gamete. If the zygospore be crushed, a thick hyaline wall will be found 

 to have formed within the boundaries of the original membrane of the 

 zygote, and on its surface a protuberance of the same material of vary- 

 ing size may often be seen which apparently corresponds to the beak just 

 mentioned (Figure 14). 



Summary of Homothallic Forms. 



The above five types of the homothallic group have been examined 

 by the writer and, with the exception of Spinellus, all have been grown 

 in pure cultures for several years. Although the figures and descrip- 

 tions of many of the species reviewed under the Citations in Part I 

 would indicate that these were also homothallic, yet in several instances, 

 as in Phycomyces and Rhizopus, the account is shown by the results of 

 the present paper to have been inaccurate as regards the thallic condition. 

 The writer does not feel justified, therefore, in including in the homo- 

 thallic group any forms, w'ith the exception of Zygorhynchus heterogamus, 

 which have not been examined with the distinct purpose of determining 

 the thallic condition present. 



If we compare the members of the homothallic group as thus limited, 

 we shall find that they offer varying degrees of differentiation between 

 the hyphae bearing zygospores and those bearing sporangia. In Spin- 

 ellus, the zygospores are borne on a distinct spiny aerial mycelium. 



