1907.] Structural Constituents of the Nucleus, etc. 449 



after fertilisation. Hence the reason for the exclusion of the extra sperms 

 must be sought in another direction. It seems probable that the cause of 

 exclusion is connected with a sudden chemical change immediately resulting 

 from the entrance of the first sperm into the cytoplasm of the egg. I shall 

 show that in one case, at any rate, there is direct evidence of such a change 

 in the reaction of the egg, and it may be remarked that it is quite intelligible 

 that such a reaction should be affected or even arrested by anaesthetics, or other 

 substances that interfere with the proper metabolism of the cell. 



In the first place, it is certain that sperms are attracted to the egg by 

 specific substances which are excreted from it and diffuse into the surrounding 

 medium. Different sperms are affected by different substances, and what will 

 attract those of one plant may be entirely without influence on those of 

 another. For example, if two vaccine tubes, filled with very weak solutions 

 of sugar and malic acid respectively, be placed in a vessel containing a mixture 

 of the actively motile sperms of a moss and of a fern, those of the former will 

 crowd into the tube containing the cane sugar, those of the fern into that 

 filled with the solution of malic acid. 



Since, then, the swarming of the sperms towards the egg is effected as 

 a response to a definite chemiotactic stimulus, it is inherently probable that 

 a correspondingly negative or repellant stimulus provides the egg with the 

 means securing it against the entrance of more than one sperm. We have 

 been able to show that this is the case in several of the Fucacese. If the 

 large eggs of Halidrys be allowed to extrude into sea-water together with the 

 sperms, the latter will, very shortly, be found to congregate on the surface of 

 the egg. Each sperm possesses two cilia, with one of which it holds on to the 

 egg, whilst with the other it lashes the water, endeavouring to force an 

 entrance through the limiting layer of the protoplasm. So active are the 

 movements that the relatively huge egg is seen to be rocked about in the 

 water. In a short time a sudden change takes place in the egg. It swells, 

 and then in a moment it becomes rugose or prickly first at one spot, and 

 thence rapidly over its whole surface. At the same instant, all the sperms 

 that were moving over its periphery rapidly leave it, and those that are 

 unable to detach themselves sufficiently quickly are seen to become 

 immediately paralysed and to die. Soon afterwards the egg shrinks in size 

 and becomes quite smooth again. Later on a membrane is excreted over its 

 surface, proving definitely that fertilisation has occurred. 



^n this instance, then, the case for the emission of a repellant, and even 

 lethal, substance from the surface of the egg as the result of fertilisation is 

 quite clear, and it perhaps diffuses from the fine mucilage-like threads that 

 are shot out from the rugose periphery. Since making these observations on 



