[Illustration: Fig. 145. Œdogonium undulatum, with oogonia and dwarf males; the upper oogonium at the right has a mature oospore.] In the oogonium wall there can frequently be seen a rift near the middle of one side, or near the upper end. This is the opening through which the spermatozoid entered to fecundate the egg. =313. Dwarf male plants.=—In some species there will also be seen peculiar club-shaped dwarf plants attached to the side of the oogonium, or near it, and in many cases the end of this dwarf plant has an open lid on the end. =314. Antheridium.=—The end cell of the dwarf male in such species is the _antheridium_. In other species the spermatozoids are developed in different cells (antheridia) of the same thread which bears the oogonium, or on a different thread. [Illustration: Fig. 146. Zoogonidia of œdogonium escaping. At the right one is germinating and forming the holdfasts, by means of which these algæ attach themselves to objects for support. (After Pringsheim.)] =315. Zoospore stage of œdogonium.=—The egg after a period of rest starts into active life again. In doing so it does not develop the thread-like plant directly as in the case of vaucheria and spirogyra. It first divides into four zoospores which are exactly like the zoogonidia in form. (See fig. 152.) These germinate and develop the thread form again. This is a quite remarkable peculiarity of œdogonium when compared with either vaucheria or spirogyra. It is the introduction of an intermediate stage between the fertilized egg and that form of the plant which bears the sexual organs, and should be kept well in mind. =316. Asexual reproduction.=—Material for the study of this stage of œdogonium is not readily obtainable just when we wish it for study. But fresh plants brought in and placed in a quantity of fresh water may yield suitable material, and it should be examined at intervals for several days. This kind of reproduction takes place by the formation of _zoogonidia_. The entire contents of a cell round off into an oval body, the wall of the cell breaks, and the zoogonidium escapes. It has a clear space at the small end, and around this clear space is a row or crown of cilia as shown in fig. 146. By the vibration of these cilia the zoogonidium swims around for a time, then settles down on some object of support, and several slender holdfasts grow out in the form of short rhizoids which attach the young plant. [Illustration: Fig. 147. Portion of thread of œdogonium showing antheridia.] [Illustration: Fig. 148. Portion of thread of œdogonium showing upper half of egg open, and a spermatozoid ready to enter. (After Klebahn).]