252 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



the two nuclei which conjugate in fertilization. But these two 

 nuclei ' are not different in nature,' as Strasburger says, and as 

 I fully believe. They cannot differ in kind, for they both con- 

 sist of germ-plasm belonging to the same species of animal or 

 plant ; and there cannot be any deeper contrast between them 

 such as would correspond to the differences between mature 

 individuals. They cannot, from their essential nature, exercise 

 any special attraction upon each other, and when we see that 

 sperm-cell and egg-cell do nevertheless attract each other, as 

 has been shown in both plants and animals, such a property 

 must have been secondarily acquired, and has no other signifi- 

 cance than to favour the union of sexual cells — an arrangement 

 which may be compared to the vibrating flagellum of the 

 spermatozoon or the micropyle of the ^gg, but which is not 

 fundamental, and is not based upon the molecular structure 

 of the germ-plasm. In lower plants, Pfeffer has proved that 

 certain chemical stimuli emanate from the ^gg and attract the 

 spermatozoid ; and according to Strasburger, the synergidae 

 in the upper part of the embryo-sac of Phanerogams secrete 

 a substance w^hich is capable of directing the growth of the 

 pollen-tube towards the egg-cell. In animals it is only known 

 as yet that spermatozoa and ova do attract each other, so that 

 the former find the latter and bore their way through its mem- 

 branes. It has also been shown that the substance of the 

 egg-bod3^ moves tow^ards the penetrating spermatozoon (' cones 

 d' exsudaiion ' in Asteridae : Folj ; and that it sometimes enters 

 upon convulsive movements {Petroniyzoti). Here therefore a 

 mutual stimulation and attraction must exist ; and perhaps we 

 must also assume that there is an attraction between the two 

 conjugating nuclei, for we cannot readily understand how the 

 cytoplasm alone could direct the one to the other, as Stras- 

 burger supposes. According to Strasburger's hypothesis, we 

 must suppose that part of the specific cytoplasm of the sperm- 

 cell continues to surround the nucleus after it has penetrated 

 into the body of the ^gg. But however this may be, the assumed 

 attraction between the conjugating nuclei certainly cannot de- 

 pend upon the molecular structure of their germ-plasm, which 

 is the same in both, but it must be due to some accessory 

 circumstance. If it were possible to introduce the female 

 pronucleus of an ^gg into another ^gg of the same species, 



