ATKINSON: THE GENUS ENDOGONE .15 



of Endogone presages the beginning of a new structure, not more so 

 than does the zygote of Piptocephalis and of certain species of Empusa, 

 etc., but it is still homologous with the zygote of the other phyco- 

 mycetes. 



In the Mucorales, aside from the meiotic divisions of the fusion 

 nuclei, the process of sporulation in the germ sporangium (sporangium 

 formed on germination of the zygote) is the same as that in the "asex- 

 ual" sporangium. The germ sporangium and the asexual sporangium 

 are morphological equivalents. The germ sporangium is not a new 

 morphological structure, though the zygote and early stages of its 

 germination presage the origin of a new morphological structure. 

 Historical experience turns it quickly back into the well-worn trail. 

 The very simple primordium of the new structure does not mark out a 

 new path until the accumulation of new experiences, together with 

 environment, provide the threshold for progress to the new structure. 



If nuclear fusion does not occur in the azygotes of Endogone then 

 the chromosome history, so far as we know, would run from generation 

 to generation without change. The two situations may be repre- 

 sented thus: 



2X. 



zygote 



->lx-> 

 azygote 



IX— > IX etc. 

 spores, mycelium. 



IX— > IX etc. 

 spores, mycelium. 



Sexual species of Endo- ( ix— >ix— >ix— >ix 



go?te. \ Mycelium and gametangia 



Parthenogenetic species f ix— >ix— >ix— >ix 



of Endogone. \ Mycelium and gametangium 



As sporulating organs the zygote and azygote (or germ sporangium 

 of the same) of Endogone are homologous structures. The true asexual 

 sporangium has been eliminated. Likewise, in the protoascomycetes, 

 where the threshold from the phycomycetes has been crossed, the 

 zygote ("germ sporangium," a generalized ascus) of Dipodasciis, and 

 the azygote ("germ sporangium," "generalized ascus") of Ascoidea, 

 as sporulating organs are homologous structures. Ascoidea is prob- 

 ably parthenogenetic, the "generalized ascus" being a transformed 

 gametangium. Free cell sporulation occurs in both genera. . This 

 interpretation of the relation of the free cell sporulating organs of 

 Dipodascus and Ascoidea is supported by the situation in Eremascus 

 fertilis and Endomyces magnusii where the ascus is in some cases of 

 sexual origin, in others of parthenogenetic origin, a single gametange 

 becoming the ascus. Endogone, with several sexual species having 

 sexually produced sporulating organs (zygotes), and other species 

 with parthenogenetically produced sporulating organs (azygotes), is 

 interesting in that it illustrates the homology of these structures, and 

 suggests how the parthenogenetic sporulating organs (generalized 

 asci) of Ascoidea, Protomyces, etc., may have arisen. It is interesting 



