442 History of the Sexual Theory. [BOOK in. 



The question was first set at rest in the Algae, where the pro- 

 cess of fertilisation could be seen directly and without exposing 

 the objects to destructive influences. That sexual propaga- 

 tion occurs in the Algae also had seemed probable, since 

 Decaisne and Thuret in 1845 discovered organs in species 

 of Fucus, and Nageli in 1846 in Florideae, which scarcely 

 admitted of any other explanation. Alexander Braun also had 

 called attention to the formation of two kinds of spores in a 

 large number of freshwater Algae. But as yet there was only 

 conjecture. Then Thuret proved by experiment in 1854, that 

 in the genus Fucus the large egg-cells must be fertilised by very 

 small swarming spermatozoids, in order to set up germination ; 

 both organs can be collected separately and in numbers in this 

 genus, and be brought together at pleasure ; Thuret even 

 succeeded in obtaining hybrids. Pringsheim first observed in 

 1855 the formation of spermatozoids in the little horns of 

 Vaucheria and established the fact that spores capable of germ- 

 ination are not formed unless the spermatozoids approach the 

 egg-cell. To Thuret's statements he added the very important 

 one, that the remains of spermatozoids may be recognised on the 

 surface of the contents of the fertilised egg-cell of Fucus, which 

 is already surrounded by a membrane. About the same time 

 Cohn published his observations on Sphaeroplea annulina, 

 which confirmed the fact of the approach of the spermatozoids 

 to the egg-cells, which consequently, as in Fucus and Vau- 

 cheria, form a cell-wall and are rendered capable of further 

 development. 



Still the decisive observation had not yet been made ; no 

 one had yet seen how the two fertilising elements behaved at 

 the moment of fertilisation. Pringsheim had the good fortune 

 to make this observation in one of the commonest of fresh 

 water Algae, Oedogonium. There he saw the moving sperma- 

 tozoid first come into contact with the protoplasmatic substance 

 of the egg-cell, and then force its way into it, blend with it 

 and dissolve. And thus the first observation was made, which 



