290 INTRODUCTION TO CYTOLOGY 



and after the entrance of the antheridial nuclei about 100 sexual fusions 

 occur. In Albugo Candida {Cystopus candidus) (Wager 1896; Davis 

 1900), Peronospora parasitica (Wager 1900), Albugo tragopogonis and 

 A. ipomceae (Stevens 1901) the mature egg has but one nucleus, which 

 fuses with a single male nucleus discharged into the egg by an antheridium 

 (Fig. 115, B). In all cases an oospore results. 



In the Saprolegniales, as shown by the researches of Davis (1903, 

 1905), Miyake (1901), Trow (1895-1905), and Claussen (1908), there are 

 two general conditions. In Saprolegnia (Trow, Davis, Claussen) from 

 10 to 15 uninucleate eggs are formed within an oogonium. One or more 

 antheridia send in conjugating tubes and deliver a male nucleus to each 

 egg, in which a single sexual fusion then occurs. In Pythium (Trow 1901 ; 

 Miyake 1901) a single uninucleate egg is produced, the fertilization 

 process closely resembling that in Albugo Candida. 



In the Zygomycetes, represented chiefly bytheMucoracese, the sexual 

 process consists in the union of the contents of two similar (except oc 

 casionally in size) multinucleate gametangia, the result of the fusion 

 being a zygospore. As shown by Blakeslee (1904) these two gametangia 

 are borne on the same mycelium in some species ("homothallic" species), 

 whereas in other species ("heterothallic" species) they are regularly 

 borne on different mycelia, no zygospores being formed in the latter spe- 

 cies on a mycelium arising from a single spore. Owing to the extremely 

 minute size of the nuclei their behavior at these stages is not well known. 

 By some investigators (Macormick on Rhizopus nigricans, 1912) it is 

 held that only one fusion occurs, the remaining nuclei degenerating. 

 Others (Keene on Sporodinia grandis, 1914) think it probable that al- 

 though some degeneration occurs, the nuclei nevertheless fuse. in pairs 

 in considerable numbers. Until further researches have been carried out 

 very little of a definite nature can be said concerning the nuclear history 

 of the Zygomycetes. 



In the Ascomycetes (see Atkinson 1915) the fusion of two nuclei 

 in the ascus was first described for several species by Dangeard (1894) 

 (Fig. 116, A), who regarded it as a sexual fusion and the ascus as an oogo- 

 nium. The matter soon became complicated when a number of cytolo- 

 gists, beginning with Harper (1895 etc.), found what they believed to be 

 a nuclear fusion at an earlier stage in the life cycle. This fusion was 

 described as occurring (a) in the archicarp when fertilized by the contents 

 of an antheridium (Harper on Sphcerotheca castagnei, 1895, 1896, Erisiphe 

 1896, Pyronema confluens 1900, and Phyllactinia 1905; Blackman and Fra- 

 ser on Sphcerotheca 1905; Claussen on Boudiera 1905) ; (6) in the archicarp 

 when the antheridium is functionless or absent (Blackman and Fraser 

 on Humaria granulata 1906; Fraser on Lachnea stercorea 1907; Welsford 

 on Ascobolus furfuraceus 1907; Dale on Aspergillus repens 1909); or (c) 

 in the vegetative cells when the archicarp is functionless or absent (Fraser 



