UNION OF THE GERM-CELLS 



197 



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 



I — 





Fig. 100. — Entrance of th^ spermatozoon into the egg. A-G. In the sea-urchin, Toxopneustes. 

 H. In the medusa, Alitrocoma. [Metschxikoff.] /. In the star-fish Asterias. [FOL.] 



A. Spermatozoon of Toxopneustes, X 2000; a. the apical body, n. nucleus, w. middle-piece, 

 /. flagellum. B. Contact with the egg-periphery. C. D. Entrance of the head, formation of the 

 entrance-cone and of the vitelline membrane (z/), leaving the tail outside. E. F. Later stages. 

 G. Appearance of the sperm-aster (j) about 3-5 minutes after first contact; entrance-cone break- 

 ing up. H. Entrance of the spermatozoon into a preformed depression. /. Approach of the 

 spermatozoon, showing the preformed attraction-cone. 



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 



