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 
J 
Fic. 17.—Ulothrix zonata.—(After Dodel-Port.) 
A, young plant. B, Zodsporangia, 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 periad 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 zodspores, 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 
