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 N�geli 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