200 



INTRODUCTION TO CYTOLOGY 



represented by the uninucleate or multinucleate contents of the anther- 

 idium, some or all of which is discharged into the oogonium through a 

 copulating tube. In Albugo Candida and Pyihium Debaryanum all but 

 one of the nuclei originally present in the young oogonium retreat to the 

 periplasm, which becomes sharply set off from the central ooplasm with 

 the single functional nucleus (Fig. 117). A dense cytoplasmic mass 

 known as the "ccenocentrum" may appear about the nucleus or in its 

 vicinity. In Saprolegnia the multinucleate protoplasm of the oogonium 

 becomes divided up by one or more vacuoles into several eggs, each of 

 which is at first multinucleate and finally uninucleate. In Mucor 

 and the other zygomycetes the multinucleate contents of two gametangia 

 behave as gametes. 



Fig. 117. — Gametogenesis and syngamy in Pythium Debaryanum. 1, formation of 

 oogonium at end of hypha. 2, oogonium and antheridium in contact. 3, egg mature; 

 degenerating nuclei in periplasm; antheridium above, with one functional and several 

 non-functional nuclei. 4> sexual nuclei in contact. {After Miyake, 1901.) 



The zoospores and motile male gametes in the phycomycetes are 

 formed by the subdivision of multinucleate protplasm. In Saprolegnia, 

 for example, the protoplasm in the swollen tip of a hypha rounds up into 

 uninucleate portions by an extension of the vacuole system. These units 

 become terminally biciliate zoospores. At a later stage of the cycle, 

 Saprolegnia produces also reniform, laterally biciliate zoospores. Con- 

 nection of the cilia with a granule (centrosome?) at the apex of a top- 

 shaped central body in the nucleus has been described for Leptolegnia 

 (A. C. Mathews, 1932). 



In the large sporangia of the zygomycetes the non-motile spores are 

 formed by a progressive furrowing of the multinucleate mass, as already 

 described (p. 165). 



The formation of spores exogenously is illustrated by conidia and 

 basidiospores. The former are cut off successively by constriction from 

 the ends of certain hyphae, each of them receiving one or more nuclei 

 derived by division from the nuclei of these hyphae. The basidiospores 

 are budded off in quartets from the basidium, whose nucleus by two 



