io6 



FECUNDATION ; HETEROGAMETES. 



together with a small quantity of protoplasm, is liberated, and so comes 

 to lie in the peripheral part of the egg. The cell- wall of the oosphere 

 is then completed, and the end of the fertilization-tube remains firmly 

 attached to it." Although the presence of the male nucleus, while in 

 the periphery of the egg, was not clearly demonstrated, yet this is not 

 absolute proof to the contrary. " I have," Trow continues, " satisfied 

 myself, however, of the presence of two nuclei in the egg at all times 

 in this stage, one peripheral and one central, and the peripheral one 

 always close to the point of attachment of the fertilization-tube." 



D 



FIG. 38. Young stages of two oogonia and two egg-cells of A. 

 americana var. cainbrica. (After Trow.) 



A, section of young stage before delimination from hypha, 



showing cytoplasmic structure and nuclei. 



B, median section at stage preceding balling; f. g. n., female 



gamete nuclei ; deg . n., degenerate nuclei. 



C, egg containing one nucleus ; apex of conjugation-tube con- 



taining male nucleus has penetrated egg. 



D, egg from a 5-day culture, in which the two gamete nuclei 



are in contact ; m. f. n., male gamete nucleus. 



At a later stage obtained from a five-day culture the two nuclei are 

 found applied to each other in the center of the egg (Fig. 38, D). 

 They are in the resting condition, and about the same size, the male 

 being distinguished from the female only by its smaller nucleolus. 

 From the fact that the sexual nuclei were found side by side in a five- 

 day culture, and from an examination of many hundreds of oospores 

 from six- to eight-day cultures, it is inferred that about three days are 

 required for the complete fusion, during which time the nuclei remain 

 in the resting condition, a phenomenon of frequent occurrence among 

 thallophytes. In the oospores of nine- or ten-day cultures, which 

 have developed a well-differentiated cell-wall, only one nucleus was 

 observed. Later, during germination, the fusion nucleus divides karyo- 



