9 



FECUNDATION ; HETEROGAMETES. 



is formed in the wall fFig. 31, B). Both cavity and slit are probably 

 the result of a swelling of the wall on the side toward the lamella. 

 The two edges of the slit roll upward and downward respectively, and 

 in this way an opening is formed in the cell-wall. The next stage in 

 development is marked by the contraction and rounding up of the 

 protoplasm to form the egg, but the oogonium is still closed by the 

 lamella. The nucleus lies in the upper end of the egg, and below it 

 is the vacuole, which has become smaller. The nucleus resembles the 

 nuclei of the vegetative cells, being relatively large with a large 



FIG. 31. Fecundation in CEdogonium boscii. (After Klebahn.) 



A, spermatozoid. 



B, young oogonium, showing origin of opening in the wall and lamella beneath. 



C, oogonium just after opening. 



D-G, upper portions of fecundated eggs, showing successive stages in fusion of nuclei. 



nucleolus (Fig. 31, C). The so-called receptive spot near the upper 

 end of the egg is formed, according to Klebahn, by the withdrawal of 

 the chloroplasts and not by the collecting of a special mass of cyto- 

 plasm. Finally, the closing lamella disappears (probably by being 

 partly dissolved in water), forming an opening for the entrance of the 

 spermatozoids (Fig. 31, C, &). No part of the plasmic contents of the 

 egg is expelled on the opening of the oogonium, as has been claimed 

 by some observers. That which is expelled, to judge from Klebahn's 



