The development of the sporangia is on the whole much like that of the club mosses, and will not be examined here in detail. The microspores (pollen spores) are formed in groups of four in precisely the same way as the spores of the bryophytes and pteridophytes, and by collecting the male flowers as they begin to appear in the spring, and crushing the sporangia in water, the process of division may be seen. For more careful examination they may be crushed in a mixture of water and acetic acid, to which is added a little gentian violet. This mixture fixes and stains the nuclei of the spores, and very instructive preparations may thus be made.[11] [11] See the last chapter for details. [Illustration: FIG. 77.--Scotch pine (except _E_ and _F_). _A_, end of a branch bearing a cluster of male flowers (♂), × ½. _B_, a similar branch, with two young female flowers (♀), natural size. _C_, a scale from a male flower, showing the two sporangia (_sp._); × 5. _D_, a single ripe pollen spore (microspore), showing the vegetative cell (_x_), × 150. _E_, a similar scale, from a female flower of the Austrian pine, seen from within, × 4. _o_, the sporangium (ovule). _F_, the same, seen from the back, showing the scale (_sc._) attached to the back. _G_, longitudinal section through a full-grown ovule of the Scotch pine. _p_, a pollen spore sending down its tube to the archegonia (_ar._). _sp._ the prothallium (endosperm), filling up the embryo sac, × 10. _H_, the neck of the archegonium, × 150.] The ripe pollen spores (Fig. 77, _D_) are oval cells provided with a double wall, the outer one giving rise to two peculiar bladder-like appendages (_z_). Like the microspores of the smaller club mosses, a small cell is cut off from the body of the spore (_x_). These pollen spores are carried by the wind to the ovules, where they germinate. The wall of the ripe sporangium or pollen sac is composed of a single layer of cells in most places, and these cells are provided with thickened ridges which have to do with opening the pollen sac. We have already examined in some detail the structure of the macrosporangium or ovule. In the full-grown ovule the macrospore, which in the seed plants is generally known as the "embryo sac," is completely filled with the prothallium or "endosperm." In the upper part of the prothallium several large archegonia are formed in much the same way as in the pteridophytes. The egg cell is very large, and appears of a yellowish color, and filled with large drops that give it a peculiar aspect. There is a large nucleus, but it is not always readily distinguished from the other contents of the egg cell. The neck of the archegonium is quite long, but does not project above the surface of the prothallium (Fig. 77, _H_).