88 Elementary Biology. 



into the cavity of the sac, and from thence to the exterior, 

 where they are fertilised by sperms which have meantime 

 escaped from the spermaria. Although many sperms 

 attack each ovum, and by rheir active motion cause the 

 passive ovum to move also, yet probably only one sperm 

 fuses with the ovum (fig. 25). Immediately after the act 

 of fertilisation the ovum becomes an embryo. As the first 

 consequence of union with the male cell, the previously 

 naked ovum takes on a cell-wall and begins to germinate, 

 elongating and dividing, at first transversely, and then ver- 

 tically, or in the direction of the long axis of the embryo. 

 Very soon after fertilisation the embryo comes to rest, 

 attaching itself to some fixed body, whilst the free end 

 develops by repeated cell- division into an organism like the 

 parent. 



To recapitulate, we have in Fucus a thallus giving rise 

 to two undoubtedly sexual cells which after union produce 

 a thallus like that from which they arose. There is thus 

 exemplified in this form a certain amount of morphological 

 differentiation. We find that a distinction can be drawn 

 between the purely vegetative portion of thallus and th^ 

 purely reproductive portion. That part which has for its 

 duty the formation of cells which have to do with the main- 

 tenance of tribal life is altered in outward form and micro- 

 scopic or histological structure from the purely vegetative 

 portions of the thallus ; yet both the purely vegetative and 

 the purely reproductive branches are modifications of the 

 same type. It can scarcely be said that the thallus of 

 Fucus, from a vegetative point of view, is morphologically 

 differentiated ; since, although we find that there is an 

 approximation to a stem, from which the branches spring, 

 and to a root, yet these are not at the same time physio- 

 logically differentiated, that is to say, they do not perform 

 the functions of circulation and absorption respectively, 

 which are the chief functions of the similar parts known by 

 these terms in the higher plants. Probably every cell in the 



