FOUNDATION OF A THEORY OF HEREDITY. 247 



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

 that certain chemical stimuli emanate from the egg and attract the 

 spermatozoid ; and according to Strasburger, the synergidae in the 

 upper part of the embryo-sac of Phanerogams secrete a substance 

 which 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 membranes. It has also been shown 

 that the substance of the egg-body moves towards the pene- 

 trating spermatozoon (' cones d* exsudalioii' in Asteridae : Fol) ; and 

 that it sometimes enters upon convulsive movements QPetromyzon). 

 Here therefore a mutual stimulation and attraction must exist ; 

 and perhaps we must also assume that there is an attraction be- 

 tween the two conjugating nuclei, for we cannot readily understand 

 how the cytoplasm alone could direct the one to the other, as 

 Strasburger 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 egg. But however this may be, the assumed attraction 

 between the conjugating nuclei certainly cannot depend 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 egg into another 

 egg of the same species, immediately after the transformation of the 

 nucleus of the latter into the female pronucleus, it is very probable 

 that the two nuclei would conjugate just as if a fertilizing sperm- 

 nucleus had penetrated. If this were so, the direct proof that egg- 

 nucleus and sperm-nucleus are identical would be furnished. Un- 

 fortunately the practical difficulties are so great that it is hardly 

 possible that the experiment can ever be made ; but such want of 

 experimental proof is partially compensated for by the fact, ascer- 

 tained by Berthold, that in certain Algae (Ectocarpus and Scytosi- 

 phon] there is not only a female, but also a male parthenogenesis ; for 

 he shows that in these species the male germ-cells may sometimes 

 develope into plants, which however are very weakly 1 . Furthermore 



1 I quote from Falkenberg, in Schenk's 'Handbuch der Botanik,' Bd. II. p. 219. 

 He further states that these are the only instances hitherto known in which un- 

 doubted male cells have proved to be capable of further development when they have 

 been unable to exercise their powers of fertilization. It must be added that the two 

 kinds of germ-cells do not differ in appearance, but only in behaviour ; the female 



