UNION OF THE GERM-CELLS 145 



equally derived from botJi germ-cells ; and certainly it would be hard 

 to find more convincing evidence that the chromatin is the controlling 

 factor in the cell by which its specific character is determined. 



We noy proceed to a more detailed and critical examination of 

 fertilization. 



B. UNION OF THE GERM-CELLS 



It does not lie within the scope of this work to consider the 

 innumerable modes by which the germ-cells are brought together, 

 further than to recall the fact that their union may take place inside 

 the body of the mother or outside, a.nd that in the latter case, both 

 eggs and spermatozoa are as a rule discharged into the water, where 

 fertilization and development take place. The spermatozoa may 

 live for a long period, either before or after their discharge, without 

 losing their fertilizing power, and their movements may continue 

 throughout this period. In many cases they are motionless when 

 first discharged, and only begin their characteristic swimming move- 

 ments after coming in contact with the water. There is clear evi- 

 dence of a definite attraction between the germ-cells, which is 

 in some cases so marked (for example in the polyp Renilla) that 

 when spermatozoa and ova are mixed in a small vessel, each ovum 

 becomes in a few moments surrounded by a dense fringe of sperma- 

 tozoa attached to its periphery by their heads and by their move- 

 ments actually causing the ovum to move about. The nature of the 

 attraction is not positively known, but Pfeffer's researches on the 

 spermatozoids of plants leave little doubt that it is of a chemical 

 nature, since he found the spermatozoids of ferns and of Selaginella 

 to be as actively attracted by solutions of malic acid or malates (con- 

 tained in capillary tubes) as by the substance extruded from the 

 neck of the archegonium. Those of mosses, on the other hand, are 

 indifferent to malic acid, but are attracted by cane-sugar. These 

 experiments indicate that the specific attraction between the germ- 

 cells of the same species is owing to the presence of specific chemical 

 substances in each case. There is clear evidence, furthermore, that 

 the attractive force is not exerted by the egg-nucleus alone, but by 

 the egg-cytoplasm ; for, as the Hertwigs and others have shown, 

 spermatozoa will readily enter egg-fragments entirely devoid of a 

 nucleus. 



In naked eggs, such as those of some echinoderms, and coelen- 

 terates, the spermatozoon may enter at any point ; but there are 

 some cases in which the point of entrance is predetermined by the 

 presence of special structures through which the spermatozoon 



