ZYGOMYCETES 111 



between the gametangia is dissolved, the nuclei undergo a division almost 

 simultaneously; their cytoplasm intermingles (Fig. 67, 2) and their 

 nuclei subsequently pair and fuse. Those without mates, especially 

 those near the periphery, degenerate and disappear. Meanwhile at 

 the surface, a wall of several layers has been formed, the suspensors 

 collapse and the zygospores presently lie free upon their substrate (Fig. 

 67, 3 and 4.) 



It is characteristic of this process that no dynamic differentiation 

 occurs between the + and — energids in spite of their separation in 

 space. Thus both gametangia are cytologically equivalent, and fertili- 

 zation is isogamous with reference to the nuclei. In Sporodinia, as in 

 the higher Oomycetes, there is no individualization of gametes, so that 

 two coenocytic gametangia copulate and accomplish, as in Albugo Bliti, 

 a multiple nuclear fusion, i.e., several fertilizations. Thus the zygospore 

 of Sporodinia is not a simple zygote like that of Olpidium but is the prod- 

 uct of two coenogametes, a coenozygote or zygosporangium. 



All other Mucoraceae so far investigated conicide with Sporodinia 

 grandis in their essential cytological relationships, e.g., Mucor (Dangeard, 

 1906; Moreau, 1914), Rhizopus, Absidia, Phycomyces and Zygorhynchus 

 (Moreau, 1914, contradicted by Gruber, 1912). In Zygorhynchus Dan- 

 geardi, all gamete nuclei but four degenerate in the young zygote. The 

 surviving nuclei fuse in pairs very late, after the endospore has been 

 formed. A similar retardation of caryogamy has been observed in Phyco- 

 myces nitens (Burgeff, 1915), in which the nuclei in the zygospores, 5 

 months old and ready to germinate, still he arranged in pairs. Perhaps 

 tendencies similar to those which cause a retardation of nuclear fusion 

 in the oospores in the Peronosporeae are present in the Mucoraceae. In 

 spite of external heterogamy, Absidia and Zygorhynchus copulate just 

 as Sporodinia, and hence are dynamically isogamous and partially 

 homothallic. 



The wall of the zygospores in the more carefully studied species of 

 Mucor, Sporodinia and Zygorhynchus consists of five layers (Vuillemin, 

 1904). The innermost is thin and granular; it forms the transition from 

 the protoplasm and, to a certain extent, is the mother layer. The next is 

 thickest and called the cartilaginous layer on account of its elasticity. 

 This is covered by a thin sheath, the middle cuticular layer. The fourth, 

 or carbonaceous layer, is fragile and brown or black; the outermost cuti- 

 cular layer is either pale and elastic, or dark and fragile, and often inter- 

 rupted or fractured. The greatest modifications in the various genera 

 are shown by the relief of the carboniferous layer, which is verrucose or 

 reticulate. The two outer layers are grouped as exospore, the three 

 inner as endospore. 



In Absidia and Phycomyces, the zygospores are loosely surrounded by 

 echinulate branches of the suspensors. In Mortierella, these branches 



