[Illustration: Fig. 305. Male prothallium of fern (niphobolus), showing opened and unopened antheridia; section of unopened antheridium; spermatozoids escaping; spermatozoids which did not escape from the antheridium.] =556. Antheridia.=—If we search among the rhizoids we see small rounded elevations as shown in fig. 297 or 305 scattered over this portion of the prothallium. These are the antheridia. If the prothallia have not been watered for a day or so, we may have an opportunity of seeing the spermatozoids coming out of the antheridium, for when the prothallia are freshly placed in water the cells of the antheridium absorb water. This presses on the contents of the antheridium and bursts the cap cell if the antheridium is ripe, and all the spermatozoids are shot out. We can see here that each one is shaped like a screw, with the coils at first close. But as the spermatozoid begins to move this coil opens somewhat and by the vibration of the long cilia which are on the smaller end it whirls away. In such preparations one may often see them spinning around for a long while, and it is only when they gradually come to rest that one can make out their form. [Illustration: Fig. 306. Section of antheridia showing sperm cells, and spermatozoids in the one at the right.] [Illustration: Fig. 307. Different views of spermatozoids; in a quiet condition; in motion (Adiantum concinnum).] [Illustration: Fig. 308. Archegonium of fern. Large cell in the center is the egg, next is the ventral canal cell, and in the canal of the neck are two nuclei of the canal cell.] [Illustration: Fig. 309. Mature and open archegonium of fern (Adiantum cuneatum) with spermatozoids making their way down through the slime to the egg.] [Illustration: Fig. 310. Fertilization in a fern (Marattia). _sp_, spermatozoid fusing with the nucleus of the egg. (After Campbell.)] =557. Archegonia.=—If we now examine closely on the thicker part of the under surface of the prothallium, just back of the “sinus,” we may see longer stout projections from the surface of the prothallium. These are shown in fig. 297. They are the archegonia. One of them in longisection is shown in fig. 308. It is flask-shaped, and the broader portion is sunk in the tissue of the prothallium. The egg is in the larger part. The spermatozoids when they are swimming around over the under surface of the prothallium come near the neck, and here they are caught in the viscid substance which has oozed out of the canal of the archegonium. From here they slowly swim down the canal, and finally one sinks into the egg, fuses with the nucleus of the latter, and the egg is then fertilized. It is now ready to grow and develop into the fern plant. This brings us back to the sporophyte, which begins with the fertilized egg. Sporophyte.