82 FECUNDATION ; HKTEROGAMETES. 



In var. crassiscpta, whose cells are smaller (narrower) and with 

 fewer nuclei, the process of cleavage differs somewhat. The eggs in 

 this variety contain, as a rule, only one nucleus. When the protoplasm 

 of the oogonium has become frothy, as described for var. braunii, 

 cleavage planes are formed at right angles to the long axis of the cell, 

 thus separating the contents into a row of short segments. 1 Here the 

 cleavage follows in such a way that a nucleus will be included in each seg- 

 ment of the cell, although in exceptional cases two nuclei maybe included 

 in a segment. In var. braunii we have, therefore, to do with multinu- 

 cleated eggs, while in var. crassisepta each egg-cell is uninucleate. 



When the egg-cells are mature, small openings are formed in the 

 wall of the oogonium through which numerous spermatozoids enter 

 (Kny, Wandtafel, LXIV). The manner in which the spermatozoids 

 unite with the cytoplasm of the egg was not observed by the authors 

 cited. According to Klebahn ('99) the fecundated egg is readily dis- 

 tinguished by its delicate membrane and by the presence of the sperm 

 nucleus which appears always in sharp contrast to the nuclei of the egg 

 (these resemble vegetative nuclei) as a small, densely staining body 

 about the size of the nucleolus (t.'e., about one micron in diameter) 

 (Fig. 29, A, B). In eggs just fecundated the sperm nucleus lies at the 

 surface beneath the delicate membrane. After a time, the length of 

 which was not determined, the sperm nucleus passes into the interior 

 of the egg, and finally fuses with one of its nuclei (Fig. 29, C, D, E). 

 Before actual fusion the two sexual nuclei remain side by side some 

 time, a phenomenon of very frequent occurrence in the plant kingdom, 

 during which the male nucleus increases in volume, its chromatic sub- 

 stance assuming the form of larger and more distinct granules, until 

 finally the two sexual nuclei can scarcely be distinguished one from the 

 other. The fusion nucleus is easily recognized by its coarsely granular 

 contents, while the other nuclei in the egg appear pale, with a few small 

 granules arranged along the nuclear membrane (Fig. 29, F). 



From the foregoing it will be seen that in Sphceroplea annulina var. 

 braunii, although several nuclei are present in the egg, fecundation 

 consists in the fusion of the spermatozoid nucleus with only one nucleus 

 of the egg-cell. Whether there exists among the several nuclei of the 

 egg any preference in the union with the male nucleus is not known, as 

 there seems to be nothing in the position or appearance of the nuclei 

 which might suggest a preference. The nuclei are irregularly grouped 

 or distributed in the cytoplasm of the egg, and it seems to be purely a 

 matter of chance as to which one will fuse with the sperm nucleus. 



1 See Kny't Wandtafel, LXIY. 



