WITH ISrOTES OiN OTIIEK SPECIES. 79 



growing filaments (Fig- 12). The number of spores formed in a sporangium depends, 

 then, on its size and the thickness of its protoplasmic lining. In an undetermined 

 species of SaproJegnia I have estimated as nearly as possible the number of 

 zoos])ores produced in each of two sporangia of different sizes, with protoplasm of 

 about average thickness. One 270,a long by 2G,v. in average diameter gave rise to 

 about 120 spores, while from one 373 X 32// about 250 spores escaped. A compari- 

 son shows that the volumes of these two sporangia bear almost precisely the same 

 relation to each other as the numbers of zoospores formed in them. 



After remaining encysted for a few hours, the zoospore begins to show renewed 

 activity. A small, circular perforation is made in the enclosing wall, and the con- 

 tents begin to emerge in the form of a small papilla, which gradually enlarges until 

 finally the entiie ])rotoj)lasmic mass lies outside of the cast-off membrane (Fig. 11). 

 The mass soon takes the form of a biciliate zoospore, and after some preliminary 

 efforts, darts away and swims freely about. In this second active stage the spore 

 has exchanged its original form for that of a kidney bean, its cilia being attached at 

 the lateral depression which corresponds to the hilum of the bean (Fig. 11, z'). At 

 the point of attachment the protoplasm is, as in the first form, h3^aline. The cilia are 

 of unequal length, and the shorter is dii-ected forward, the other backward, during 

 the swarming. It is an interesting fact that this form of the zoospore corresponds 

 with those of the related Peronosj^oraceoe, and with those of some of the A>icylistacem. 

 It would seem that this must be i-egarded as the primitive zoospore of the Phycomy- 

 cetes, as Ward has suggested ('83), the form with apical cilia being a secondary'- one 

 acquired within the limits of the present family. After half an hour or more of 

 activity, the spore again settles down and becomes encysted as before. 



In most of the genera now under discussion, this double swarming or dhplaaetism 

 of the zoospores is the rule, although in exceptional spores the second swarming may 

 be omitted without apparent influence upon their germinating power (Fig. 11, a). It 

 would seem that the object of the first swarming is the emptying of the sporangium, 

 and that of the second the distribution of the spores, to enable them to reach new 

 sources of food sui)ply; since, as Pfeffer ('84) has shown, they are strongly attracted 

 by various organic substances. Although the first swarming accomplishes both 

 ends to a consideiable degree in Saprolegnia, it does not permit the wide distribution 

 which the longer second period allows. The zoospores of the secon.d form also seem 

 to range over a much wider region and to be more actively locomotive than those 

 of the first. The condition which is exceptional in most of these genera has 

 become permanent in Pythiopsis, which ma}^ I'epresent a reduced Saprolegnia, that, 



A. p. S. — VOL. XVII. K. 



