Page:Popular Science Monthly Volume 62.djvu/314

308 Among the Chaetophoraceae is the genus Aphanochaete (see Fig. 4), presenting one of the most interesting stages of sexual evolution and bridging over a very important gap. Aphanochaete develops its female gamete singly in a mother cell. It is discharged into the water surrounded by a delicate vesicle. The gamete, although very large, is ciliate. However, it moves about scarcely at all, and does not leave the vesicle. The sperms gather around the dissolving vesicle, and finally one pierces it and fuses with the female gamete. The fertilized egg immediately begins to turn on its axis and moves about in the water for a few moments and then settles down to rest. It is probable that the cilia remain on the egg for this short period of motility, but it is evident that the female gamete has entirely given up the free-swimming habit. But what is more important, the oogonium appears to deliver its gamete with reluctance, for it is not entirely freed from its investing membrane

until after fertilization. It would be but a small step for Aphanochaete to retain this female gamete in the oogonium as a motionless egg and thereby present the furthest extreme of heterogamy. Such a condition would place Aphanochaete very close to Coleochaete, which it strikingly resembles in some important respects.

The last form in this series of green algae is Coleochaete, the sole representative of the family Coleochaetaceae. Of all the algae this type probably stands the closest to the liverworts, not because its sexual organs are similar, but because it presents a sphorophyte generation resembling that of the lowest liverworts (Ricciales). Coleochaete is heterogamous, but its sexual organs can scarcely be compared with the archegonia and antheridia of the Bryophytes. These structures are to be related only with the greatest difficulty to the sexual organs of the algae, and probably not through any existing type of structure, unless it be the organ called the plurilocular sporangium.