LABOULBENIALES 393 



uals" only result when for some reason the perithecium aborts, and an abnormal 

 multiplication of antheridia sometimes results as a teratological phenomenon. The 

 "female plants" always have antheridia in the usual position. 



The simple antheridia seem to have little in common with the conidial forms of 

 Thielavia and Pyxidiophora, either in appearance or ontogeny, although no cytological 

 studies have been undertaken on either group. 



Summary. — The derivation of Ascomycetes from Zygomycetes, which 

 forms the basis of our treatment, is built on the following conceptions. 

 In the Zygomycetous series there are present the beginnings of a three- 

 fold development. In the first place, in the privileging of sexual nuclei : 

 all the gametangial nuclei are no longer activated as sexual nuclei or, in 

 any case, do not all take part in the sexual act, but the sexual function is 

 fulfilled by a small portion of them and finally by only one gametangial 

 nucleus (Endogone). Secondly, in the Zygomycetes there appears the 

 beginning of a delay in caryogamy which no longer normally takes place 

 in the gametangium, but in outgrowths of it (Phycomyces, Endogone). 

 And thirdly, the zygospore germinates increasingly by a germ sporangium 

 (Polyphagus, Phycomyces) rather than a germ tube. 



According to the theory presented here, these three tendencies are 

 realized in the lowest Ascomycetes. Dipodascus differs from the Endo- 

 gone only in forming a sporangium instead of a zygote as the product of 

 the sexual act. This difference, however, may rest chiefly on biological 

 relationships. In Dipodascus, as in Endogone, two coenocytic copulation 

 branches enter into open communication. As in Endogone lactiflua, how- 

 ever, only one gametangial nucleus from each gametangium participates 

 in each sexual act, so that all other gametangial nuclei are superfluous. 

 While in Endogone the supernumerary nuclei soon disintegrate, in 

 Dipodascus they are retained longer. Thus between Endogone and 

 Dipodascus there is a difference like that between the Pythium-Phytoph- 

 thora group and the higher species of Peronospora of the Oomycetes. In 

 the latter, the supernumerary peripheral oogonial nuclei persist and are 

 used for secondary vegetative purposes. In Dipodascus the supernumer- 

 ary nuclei are used for accessory structures, apparently in connection with 

 the surplus protoplasm. 



This similarity in functional differentiation of gametangial nuclei has 

 led in both groups to similar morphological developments. In the higher 

 Oomycetes, the egg cells were differentiated by free cell formation from 

 the oogonial protoplasm about the sexual nuclei, while the supernumerary 

 nuclei lay outside in the peripheral periplasm. Similarly in Dipodascus, 

 the spore portions about the daughter nuclei of the primary ascus nucleus 

 are cut out of the ascus protoplasm by free cell formation while the sexu- 

 ally inactive nuclei remain undivided in the periplasm. If all the nuclei 

 lying in the young ascus were equivalent, as they are in the sporangium 

 of Polyphagus, this complication would never have arisen; but the ascus 



