62 



FECUNDATION; MOTILE ISOGAMETES. 



The process of cell-formation by which the gametes are devel- 

 oped from the protoplast of the gametangium has been observed and 

 described in some detail by Dodel ('76) and by Strasburger ('92). 

 These authors agree that the gametes arise not by the process of free 

 cell-formation, as understood at the time, but by successive bipartitions 

 of the entire plasmic contents of the cell. According to Strasburger 

 ('92) the process of division in the development of the swarm-spores, 

 which is exactly the same for the gametes, differs from the beginning 

 in a very marked way from the vegetative cell-divisions. At first the 

 cell-contents undergo apparently a sort of rejuvenescence by which 

 the protoplast becomes independent of both the outer and inner plasm 



D 



FIG. 17. Ulothrix zonata. (After Dodel-Port.) 



A, young plant. B, Zoosporangia, showing escape of swarm-spores. 



C, an asexual swarm-spore. D, gametangia, showing gametes and escape of same. 



E, two gametes. F, G, copulation of gametes. 



H, zygote. J, zygote after a period of rest. 



K, germinating zygote whose contents have divided into a number of swarm-spores. 



membranes. In the first division the granular plasma only is halved. 

 The outer plasma membrane (Hautschicht) is undivided, and the 

 membrane surrounding the vacuole remains unchanged. By further 

 successive divisions these two protoplasts give rise ultimately to the 

 gametes. The process of division is the same whether gametes or 

 asexual swarm-spores result (Fig. 17, D). Strasburger has expressed 

 the opinion that, in the development of the gametes, only one more 

 cell-division is necessary above those required for the zoospores, and 

 this division renders the resulting cells or gametes incapable of further 

 independent development. In what way this last division incapaci- 

 tates the gametes for further independent development was not dis- 

 cussed at the time. That view was probably prompted by Weismann's 



