Page:Popular Science Monthly Volume 62.djvu/312

306 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, b, Ectocarpus secundus, showing the two sizes of the gametes; c, Cutleria mullifida, 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 process 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