222 SEXUAL REPRODUCTION ((EDOGONIUM) 



cells; that producing the egg (ovum) is termed the oogonium, 

 whilst that forming the spermatozoids is known as the 

 antheridium. 



In (Edogonium the oogonia are more or less oval cells which 

 may arise in any part of the filament and occur either singly or 

 in short chains (Fig. 117, h, oog.). At one point the wall of the 

 oogonium develops a small papilla by the breaking down of 

 whose tip an aperture for the entry of the male cell is created ; 

 in some species of (Edogonium, however, opening is effected by a 

 complete transverse split in the wall. The single egg, formed by 

 the contracted protoplast of the oogonium, develops a small 

 colourless area, adjacent to the aperture, known as the receptive 

 spot (Fig. 120, C, .r.s.), and at this point a quantity of mucilage 

 is extruded shortly before fertilisation. 



The antheridia are small and tabular, being formed by re- 

 peated transverse division of cells of the filament (Fig. 120, A, a.). 

 Each produces one, or more commonly two, spermatozoids (sp.) 

 which are diminutive, though almost colourless, replicas of the 

 zoospores, and which are liberated in the same manner (Fig. 120, 

 D). Should a spermatozoid in the course of its movement come 

 into the neighbourhood of an oogonium, it appears to be attracted, 

 probably by some chemical substance l in the extruded mucilage, 

 and, passing through the aperture, penetrates into the egg 

 (Fig. 1 20, C), the cytoplasm and nucleus of the one thereupon 

 fusing with those of the other. Neither sexual cell can develop 

 independently, an indication of specialisation as compared 

 with Ulothrix. 



It is customary to speak of such sexual union as fertilisation, 

 the egg being said to be fertilised by the spermatozoid. The 

 effect of the fusion of a spermatozoid with an egg is probably 

 of the nature of a chemical and physical stimulus, without which 

 further development of the ovum is impossible. This view is 

 supported by the fact that the eggs of Sea-urchins have been 

 induced to develop into embryos by mere immersion in suitable 

 solutions, whilst those of the Frog have been caused to undergo 



1 The influence exerted by chemical substances on the direction of move- 

 ment of motile elements is spoken of as chemotaxis, and the positive chemo- 

 taxis evident in sexual union is only one of many examples of such 

 chemical stimulation (cf. p. 232). 



