=317. Sexual reproduction. Antheridia.=—The antheridia are short cells which are formed by one of the ordinary cells dividing into a number of disk-shaped ones as shown in fig. 147. The protoplasm in each antheridium forms two spermatozoids (sometimes only one) which are of the same form as the zoogonidia but smaller, and yellowish instead of green. In some species a motile body intermediate in size and color between the spermatozoids and zoogonidia is first formed, which after swimming around comes to rest on the oogonium, or near it, and develops what is called a “dwarf male plant” from which the real spermatozoid is produced. [Illustration: Fig. 149. Male nucleus just entering egg at left side.] [Illustration: Fig. 150. Male nucleus fusing with female nucleus.] [Illustration: Fig. 151. The two nuclei fused, and fertilization complete. Figs. 149-151.—Fertilization in œdogonium. (After Klebahn).] =318. Oogonia.=—The oogonia are formed directly from one of the vegetative cells. In most species this cell first enlarges in diameter, so that it is easily detected. The protoplasm inside is the egg-cell. The oogonium wall opens, a bit of the protoplasm is emitted, and the spermatozoid then enters and fertilizes it (fig. 148). Now a hard brown wall is formed around it, and, just as in spirogyra and vaucheria, it passes through a resting period. At the time of germination it does not produce the thread-like plant again directly, but first forms four zoospores exactly like the zoogonidia (fig. 152). These zoospores then germinate and form the plant. =319. Œdogonium compared with spirogyra.=—Now if we compare œdogonium with spirogyra, as we did in the case of vaucheria, we find here also that there is an advance upon the simple condition which exists in spirogyra. Growth and division of the thread is limited to certain portions. The sexual organs are differentiated. They usually differ in form and size from the vegetative cells, though the oogonium is simply a changed vegetative cell. The sexual organs are differentiated among themselves, the antheridium is small, and the oogonium large. The gametes are also differentiated in size, and the male gamete is motile, and carries in its body the nucleus which fuses with the nucleus of the egg-cell. [Illustration: Fig. 152. Fertilized egg of œdogonium after a period of rest escaping from the wall of the oogonium, and dividing into the four zoospores. (After Juranyi.)] But a more striking advance is the fact that the fertilized egg does not produce the vegetative thread of œdogonium directly, but first forms four zoospores, each of which is then capable of developing into the thread. On the other hand we found that in spirogyra the zygospore develops directly into the thread form of the plant. [Illustration: Fig. 153. Tuft of chætophora, natural size.] [Illustration: Fig. 154. Portion of chætophora showing branching.]