1915] 



ATKINSON PHYLOGENY IN THE ASCOMYCETES 



319 



out into an elongate stout ' ' ascus ' ' or zygogametangium with 

 the production of numerous spores. While all phases of the 

 nuclear phenomena have not yet been made clear, the gametes 

 are multinucleate, and multiplication either of the sex nuclei, 

 or of the fusion nucleus, 

 takes place in the gen- 

 eralized ' ' ascus. ' ' This 

 so-called ascus is an 

 outgrowth of the undif- 

 ferentiated oogonium or 

 ascogonium. The split- 

 ting up of such a 

 generalized ascus by fil- 

 amentous outgrowths, 

 the ascogenous threads, 

 which branch and pro- 

 duce terminal asci con- 

 taining fewer spores, 

 would be a very natural 

 course in progressive 

 evolution, specialization, 

 and increase in spore 

 output. 



Origin of the ascus in the Endomycetaceae. — The tendency 

 of generalized forms to split up in different directions, often 

 giving rise to divergent lines or series, is a well founded prin- 

 ciple in the doctrine of descent. These series are often of 

 different character in respect to numbers and diversity of 

 forms, as well as to progression or reduction in one or more 

 structures. One of the directions in which descent from such 

 a generalized, coenocytic, germinating zygote (or ascus) as 

 represented by Dipodascus has taken place is that of reduction 

 in size of the generalized ascus and in the number of spores. 

 Evidence of this reduction is furnished by Dipodascus itself; 

 for, as the culture ages the asci become smaller and smaller 

 and the spores fewer in number. In this way by reduction in 

 number of spores to 8 and 4, just permitting the meiotic 

 nuclear divisions, forms like Eremascus and Endomyces have 



Fig. 1. Dipodascus albidus: A, copulation 

 of gametangia ; B, communication established 

 between antheridium and oogonium; C, the two 

 sex nuclei approaching each other; D, fusion 

 nucleus large, vegetative nuclei small; E, 

 growth of generalized ascus from oogonium side 

 of copulating gametes, early stages of, in C and 

 D; F, generalized ascus with numerous spores; 

 G, spore mass crowded out of end of ascus. a, 

 antheridium; o, oogonium. — A-E, after Juel; 

 F and G, after Lagerheim. 



