108 MEDICAL MYCOLOGY 



gametangia are cytologically equivalent, and fertilization is isogamous with 

 reference to the nuclei. In Sporodinia, since there is no individualization of 

 gametes, two coenocytic gametangia copulate and accomplish multiple fertiliza- 

 tions. In ZygorrhyncJius Dangcardi, all but 4 gamete nuclei degenerate in 

 the young zygote. The surviving four fuse in pairs very late after the endo- 

 spore has been formed. A similar retardation of caryogamy has been observed 

 in Phy corny ces nit ens (Burgeff 1915) in which the nuclei in zygospores 5 

 months old and ready to germinate, still lie in pairs. 



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

 Sporodinia, and ZygorrhyncJius consists of 5 layers (Vuillemin 1904). The in- 

 nermost layer 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 is 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 cuticular layer is either pale and elastic, 

 or dark and fragile, and often interrupted or fractured. The greatest modifi- 

 cations in the various genera are shown by the surface of the carbonaceous 

 layer which is verrucose or reticulate. The two outer layers are grouped as 

 the exospore, the three inner as the endospore. 



In Absidia and Phycomyces the zygospores are loosely surrounded by 

 branches from the suspensors (Fig. 9, 1, 7). In Mortierella these branches 

 intertwine with the neighboring hyphae into a solid felt whose outer surface 

 is cuticularized and brown. AVithin this tissue lies the zygospore. 



The zygospores germinate only after a long resting period. The exospore 

 is ruptured, the endospore puts forth a germ tube which develops to a 

 mycelium or, with insufficient nourishment, directly to a sporangium or a 

 conidiophore. 



During germination, meiosis of the diploid nuclei occurs. Where the germ 

 tube becomes the fundament of a sporangium (e. g., Phycomyces nitens, Burgeff 

 1915) meiosis occurs only in the latter which is called a "germ sporangium," 

 and as we shall see later is the precursor of the ascus. The sexual relationships 

 existing at meiosis have been more closely studied for three types. {Sporodinia, 

 Mucor Muceclo, and Phycomyces nitens, Blakeslee 1904, 1906.) In Sporodinia 

 the sporangiospores are liomothallic and the separation of the + and - energids 

 occurs only in the formation of the copulation branches. 



In the heterothallic Mucor Mucedo the separation of the + and - energids 

 occurs probably in the formation of sporangia ; i.e., the spores are all of one 

 sex in one sporangium, either all + or all -. 



In the equally heterothallic Phycomyces nitens, the separation of sexes 

 occurs only in the formation of spores. Even so, it is incomplete ; besides the 

 + and - spores there are also unstable, neutral, bisexual spores in whose 

 sporangia the separation into + and - spores is continued (Burgeff 1912). 



