ZOOSrORES OF THE SAPEOLEGNIEIi:. 433 



at least two zoospores, which, as they round off, become too 

 large for the calibre of the beak and retire from it. The end 

 wall of the beak, convex outwardly, is now seen in optic section 

 to be menisciform, thickened in the middle and thinning off at 

 the edges ; and it presents that peculiar brilliant lustre which 

 is so characteristic of diffluent or coUenchymatised cell walls. 

 And, indeed, it does shortly disappear, a phenomenon which De 

 Bary ascribes with great plausibility to the secretion of some 

 ferment. In some cases, especially in the undetermined 

 Saprolegnia, we may actually perceive the disappearance, 

 followed by the immediate outrush of the zoospores; but 

 usually in Achlya and Leptomitus the foremost zoospore 

 enters the beak, and closes up against its end wall, which, pos- 

 sessing the same refractive index, ceases to be distinguishable. 

 In this case the outrush of the zoospores is the sole indication 

 of the dissolution of the end wall of the beak. As soon as the 

 way is clear, the zoospores crowd to the opening, closely serried, 

 leaving a clear space along the side wall of the sporange, and 

 giving at first sight the impression that they are indeed pushed 

 by such a vis a latere as the expulsive matter of Ue Bary 

 would exert. They force their way through the opening, often 

 becoming constricted as they do so, and emerge obtusely pyri- 

 form or of a stumpy " biscuit shape," with the anterior end the 

 narrower, and possessing two flagella (tractella). The hinder 

 ones in the sporange, as room is made for them, also lose their 

 rounded or polygonal form, and assume this. In Achlya, as 

 the zoospores emerge, they remain near the entrance, grouped 

 in a hollow sphere, their narrower rounded ends turned in 

 towards the centre. Each new comer presses in between the 

 others, so that the sphere grows in size till the zoospores have 

 all settled there. 



The outrush of the spores, so rapid at first, is seen to slacken 

 after some time, and then we can note more readily the real 

 mode of procedure. A man up in a balloon, observing a crowd 

 at the doors of a theatre, might well regard the inrush of sight- 

 seers when the doors open as the expression of a vis a tergo j 

 but he can correct his judgment by observing the behaviour of 



