304 XXII. REPRODUCTION OF ALG^E. 



move for a little while in the opposite direction, but ultimately 

 they all get to the shaded side of the drop. With very weak 

 illumination a definite direction to the movement is hardly recog- 

 nisable, and the same is the case with unhealthy spermatozoids. 

 As the oospheres are specifically heavier than sea-water, the 

 apheliotropism of the spermatozoa takes them away from the 

 surface of the water, and therefore in the direction in which 

 the oospheres are likely to be. The spermatozoa can be best 

 fixed with iodine and picric acid, and show the same structure as 

 those already studied (G). 



In order to see the processes of fertilisation, we transfer the 

 olive-green mucilage from the female conceptacle to drops of sea- 

 water on a number of object-slides. We examine these in order 

 to determine when evacuated oospheres are present. Such 

 ought always to be found within the first hour, but oospheres 

 which have been evacuated for several hours are still capable 

 of fertilisation ; so that we place our preparations in a dark 

 chamber, and can use them for observation one after the other. 

 If we follow under the microscope the method of evacuation of 

 the oospheres, we shall see, in contradistinction to F. platycarpus y 

 that the envelope of the oogonium remains visible up to the 

 setting free of the oospheres, that its inner layer is specially 

 clearly marked, and that during the evacuation of the oospheres 

 the outer layer is turned over. If we now bring a little of the 

 orange-red spermatozoidal mucilage into a preparation containing 

 freed oospheres, these are quickly surrounded by spermatozoids. 

 By turning the preparation in such way that the spermatozoids, 

 avoiding the light, come into contact with the oosphere, we can 

 determine that even those spermatozoids which are removed 

 from the oosphere by a distance equal to the diameter of these 

 latter, suddenly turn from their path in order to move towards 

 the oosphere. This attraction extends over approximately twice 

 the diameter of an oosphere. As has been determined by Pfeffer, 

 this attraction depends upon a body given off from the oosphere, 

 which acts as a chemical stimulus in determining the direction of 

 movement of the spermatozoids. The spermatozoids cling to 

 the oospere, which is soon completely covered with them. The 

 spermatozoids lie obliquely on the surface of the oosphere, with 

 the beak and a portion of that side which is devoid of cilia ; while 

 the hinder, laterally inserted, cilium continues for a time to vibrate, 

 and sets the oosphere in rapid rotation. The rotation takes place 



