318 SPENCER WALPOLE AND T. H. HUXI-EY. 



to one point of its surface. After a few minutes it becomes 

 quiescent and surrounds itself with an extremely delicate 

 transparent coat. But this repose is of a very short dura- 

 tion, as it soon einerji^es froui its envelope, and moves ahout 

 even more actively than hefore. It has now an elonj^ated 

 oval shape, and has two cilia which proceed from one side of 

 the oval. This second active state may last for a day, or 

 perhaps two; and it is obvious that, from the activity of the 

 motion of the zoospores, to say nothing of accidental currents, 

 they may thus be carried a long way from the parent stock. 

 Sooner or later, however, they again come to a state of rest, 

 which is final, and they then usually germinate. That is to 

 say, one, or perhaps two, delicate filaments grow out and 

 rf- present the primitive hyphse of a new Saprolegnia (Fig. 

 IV, p. 316). 



If the spore has attached itself to some body which is 

 incapable of affording it nourishment, it may not germinate 

 at all, or, if it germinates, it speedily dies. But, if it falls 

 uj)on an appropriate soil, such, for example, as the body of 

 another dead fly, the spore sends a prolongation inwards 

 which perforates the tough chitinous cuticle of the fly, and 

 gives rise to a system of root-hyphse in its interior ; while, 

 simultaneously, it grows outwards into a similarly ramifying 

 stem-hvpha, the branches of which soon enlarge into zoo- 

 sporangia and give rise to zoospores, as before. 



The growth and development of the Saprolegnia take place 

 with extraordinary rapidity. In thirty-six hours from the 

 first infection of the body of a dead fly with the Saprolegnia 

 spores, it may be covered with a thick coat of stem-hyphEe a 

 fifth of an inch long ; and, in the course of the second or 

 third day, a thousand of these may have developed and 

 emptied their sporangia, thus setting free some 20,000 zoo- 

 spores, every one of which is competent to set up the same 

 process in a new fly-corpse. As all this production takes 

 place at the expense of the tissue of the fly, the supply of 

 nutritive material gradually diminishes. At about the fourth 

 day, or perhaps not till later, new forms of sporangia, termed 

 " dictyosporangia " (Fig. Ill, p. 316), in which the spores 

 encase themselves and often germinate while still within 

 the sporangium, make their appearance ; and the ordinary 

 zoosporangia diminish in number. Not unfrequently, about 

 this time or subsequently, the hyphae tend to break up into 

 short joints which are themselves capable of germination. 

 Finally, after the fifth or sixth day, a new kind of sporan- 

 gium usually makes its appearance, which is termed an 

 Oosporangium^ inasmuch as the spores to which it gives rise 



