FIRST GROUP. 



2. The transition from the formation of zygospores to the second mode of 

 fertilisation, the Formation of Oospores, is quite gradual, and it is complete 

 both in forms with motile and in those with non-motile gametes. We find inter- 

 mediate stages in the former case in different cycles of affinity. For instance, the 

 gametes of Ectocarpus siliculosus are perfectly alike externally, but their behaviour 

 is different ; one swarm-cell settles down and loses its cilia, and is thus transformed 

 into an oosphere with which, as we learn from Berthold, male swarm-cells coalesce. 

 In Cutlen'a, which belongs to the same group, the two motile 

 A- sexual cells differ much in size ; the male planogamete, the sper- 



matozoid, is much smaller than the female which soon comes to 

 rest, and becomes an oosphere. In Fucus the difference is still 

 greater; the female gamete, the oosphere, is here non-motile, 

 while the male, the spermatozoid, is still a motile cell ; but 

 here too the act of fertilisation takes place outside the organ in 

 which the female gamete was formed,' and which is known as 

 the oogonium. In the green Algae, on the contrary (the Chloro- 

 phyceae), the female gamete or oosphere is not only non-motile, 

 but remains lying in the oogonium (Fig. 2). 



The male elements or spermatozoids (Fig. 2), the mother-cells 



FIG. 2 . Formation of 



the oogonium, o the r i i i / 7 n j i j r 



oosphere, a the anthe- of which are termed anthendia, are very small and move by aid 01 



ridium, tn a small male ... . . i r i i j r ^-v 



plant (dwarf male), s the cilia ; they swim about seeking for the oospheres, and fertilisation 

 is effected by the coalescence of their substance with that of the 

 oospheres. The volume of matter which goes to form the spermatozoid is extremely 

 small; nevertheless by union with it the oosphere is excited to form a cell-wall 

 and becomes the oospore. 



\ , A 



FIG. 3. Examples of the formation of oogonia, the oospore being surrounded with envelope-tubes before or after 

 fertilisation. A Coteochaete, B Chara, m spermatozoid, -w oogonium (in Coleochaete with a long beak), h envelope, 

 which forms round the oogonium or oospore in Coleochaete after, in Chara before fertilisation, cs oospore. 



The oospore may germinate immediately after its formation, and produce a 

 plant like its own mother-plant, as in Fucus^ or like the zygospores it first passes 

 through a period of rest and then germinates, and this is the usual case. Here too 

 the germinating oospore may at once produce a plant like the mother-plant, as in 



