12 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



demonstrate the conjugation-pore from this aspect also. Fig. 8 shows 

 such a case in which the surface of contact of the gametes lies almost 

 exactly in the plane of the section. The conjugation-pore appears here 

 as a clean-cut opening in the floor of the antheridium. It is a perfectly 

 clear, circular area, sharply distinguished from the bluish material of 

 the cell-walls which surround it. This figure, in the region of the 

 antheridium, represents an optical section lying in the plane of contact 

 of the gametes, and the existence of an open pore from the antheridium 

 to the oogonium is unmistakable. It is evident from the description 

 given above that the shape and relations of the gametes are such that, 

 with any possible orientation of the host-leaf to the plane of the sections, 

 the aspects shown in figs. 6, 7, and 8 will be quite as common as those 

 shown in figs. 9, 10, and n. The necessity for finding this latter aspect 

 of the gametes in order to demonstrate the process of fusion with the 

 greatest ease is doubtless the reason why some observers have failed to 

 convince themselves that a union of the gametes actually occurs. Fig. 9 

 shows this latter aspect of the fusing-cells. The plane of contact of the 

 gametes cuts the plane of the section almost exactly at right angles, and 

 the conjugation-pore appears as an absolutely unmistakable open pass- 

 ageway leading from the antheridium to the oogonium; and the cyto- 

 plasmic contents of the gametes are seen to have fused to form a con- 

 tinuous protoplasmic mass through which the male nucleus has migrated 

 into the oogonium. The two pronuclei lie side by side in the oogonium 

 and are readily distinguishable by their difference in size. In this par- 

 ticular case a vacuole lies just outside the conjugation-pore in the anthe- 

 ridium, indicating, perhaps, a relative increase in the cell sap of the 

 latter after the departure of the male nucleus. 



The entire contents of the antheridial cell does not enter the oogo- 

 nium. In the old antheridium, after fertilization is complete there is 

 considerable material which degenerates into a dense, structureless mass. 

 In fact, it seems quite likely that here, as in many other cases, only a 

 minimal amount, if any, of the cytoplasm of the male cell is actually 

 taken up into the egg. The male nucleus apparently leaves the most, if 

 not all, of its cytoplasm behind when it enters the egg. The cytoplasm 

 of the antheridium is certainly less dense and is more vacuolar for a 

 time after fertilization has taken place. A few deeply-stained granules 

 are present in the antheridium while the conjugation-pore is still open 

 (fig. 9) and are perhaps the first indication of the degeneration of 

 its protoplasm. After the closing of the conjugation-pore they may 

 become more numerous (fig. 10), but the antheridial cytoplasm keeps its 

 spongy appearance for a time (fig. n). Soon the whole of the cyto- 



