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making them neutral. In other words, sex in Ectocarpus is at such 

 a low level of differentiation that its characteristics are still under 

 the potent influence of external factors, and zoospores that will be 

 sexual under certain conditions, e. g., high temperature, will germinate 

 parthenogenetically if this environment is not present. Ectocarpus 

 is then another illustration with Ulothrix and Hydrodictyon of that 

 primitive state, considered in my former paper, when the asexual 

 zoospore became the gamete under the influence of external factors. 



But when the zoospores of Ectocarpus have sexuality there is a 

 decided tendency for the smaller gametes to seek the larger more slowly 

 swimming elements and fuse with them. The small gametes have, 

 therefore, male characteristics, which is further shown by the fact that 

 they are unable to develop parthenogenetically or, if they do so, grow 

 into small plants much weaker than the normal. 



Fig. 3. a, Ectocarpus siliculous, Female Gamete Surrounded by Male, Stages in the Fu- 

 sion op Gametes, b, Ectocarpus secundus, showing the two sizes of the gametes ; c, Cutleria 

 muUiftda, male and female gametes and gametangia. (a, after Berthold; b, Sauvageau; c 

 Thuret.) 



And now we may consider a habit peculiar to certain species of 

 Ectocarpus which is of great interest as an important stage in the pro- 

 cess of sexual evolution. It has been most thoroughly studied in 

 Ectocarpus siliculosus. The female gametes of this species have a 

 limited period of motility. They swim about slowly and shortly come 

 to rest, attaching themselves. The male gametes are motile for a 

 much longer time. While at rest the female gametes attract the males 

 which hover around (see Fig. 3, a), with the result that one fuses with 

 a motionless cell and fertilizes it. This is an exceedingly interesting 



