OOMYCETES 



81 



exception of Pythium, they arise only in the tissue of the host as clavate, 

 terminal or intercalary cells of hyphae or hyphal branches; their develop- 

 ment shows ascending differentiation proceeding parallel to the lines of 

 development of conidiophores and conidia. 



In Pythium 10 to 15 nuclei enter the young oogonium, and three to six 

 the young antheridium; thereupon the swelling is abjointed from the 

 rest of the hypha. About simultaneously in both organs, the nuclei 

 undergo division (Fig. 50, 2), in which there have been counted in P. 

 ultimum (Trow, 1901) at least six chromosomes and in P. debaryanum 

 (Miyake, 1901) approximately eight. Meanwhile the protoplasm of the 

 oogonium has been differentiated more or less definitely into a thicker, 

 denser, central ooplasm and spongy periplasm. In P. ultimum, all nuclei 



Fig. 50. — Pythium debaryanum. Gametangial copulation. 1. Young oogonium and 

 antheridium. 2. Mitoses. 3. Degeneration of all but one male nuclei. 4. All but one 

 female nuclei migrate to the periphery and degenerate. 5, 6. Plasmogamy. 7. Mature 

 oospore. {After Miyake, 1901.) 



but one migrate to the periplasm and degenerate there. In P. debary- 

 anum, all migrate into the periplasm, then one returns while the others 

 degenerate (Fig. 50, 4). Meanwhile in the antheridium, at least in P. 

 debaryanum, all nuclei but one disintegrate (Fig. 50, 3); in P. ultimum 

 they all appear to remain intact. The antheridium puts forth a conju- 

 gation tube into the oogonium and allows the male nucleus (in P. ultimum 

 one of the many), together with a part of the male protoplasm, to pass 

 over into the egg cell; there it lies in the vicinity of the female nucleus 

 but only fuses with it after a long time. Shortly before nuclear migra- 

 tion, the central ooplasm of the oosphere surrounds itself with a 

 membrane, during which the greater part of the periplasm is resorbed; 

 later a thick- walled endospore is formed on its inner side; but a true epi- 

 spore, laid down by the periplasm, is not present. In both species the 

 oospore germinates by a germ tube which develops to a hypha. In 

 P. ultimum it can take place after 24 hours. In other species of Pythium 

 different germination forms have been described, as in P. vexans, zoospore 



