[Illustration: FIG. 29.--Successive stages in the division of the ovum, or egg-cell, of a worm. (After Strasburger.) _a_ to _d_ show the changes taking place in the nucleus and surrounding cell-contents, which result in the first segmentation of the ovum at _e_; _f_ and _g_ show a repetition of these changes in each of the two resulting cells, leading to the second segmentation stage at _h_.] Lastly, the only other distinction of a physiologically significant kind between a single cell when it occurs as a Protozoön and when it does so as the unfertilized ovum of a Metazoön is, that in the latter case the nucleus discharges from its own substance two minute protoplasmic masses ("polar bodies"), which are then eliminated from the cell altogether. This process, which will be more fully described later on, appears to be of invariable occurrence in the case of all egg-cells, while nothing resembling it has ever been observed in any of the Protozoa. We must now consider these several points of difference _seriatim_. First, with regard to sexual propagation, we have already seen that this is by no means the only method of propagation among the multicellular organisms; and it now remains to add that, on the other hand, there is, to say the least, a suggestive foreshadowing of sexual propagation among the unicellular organisms. For although simple binary fission is here the more usual mode of multiplication, very frequently two (rarely three or more) Protozoa of the same species come together, fuse into a single mass, and thus become very literally "one flesh." This process of "conjugation" is usually (though by no means invariably) followed by a period of quiescent "encystation"; after which the contents of the cyst escape in the form of a number of minute particles, or "spores," and these severally develope into the parent type. Obviously this process of conjugation, when it is thus a preliminary to multiplication, appears to be in its essence the same as fertilization. And if it be objected that encystation and spore-formation in the Protozoa are not always preceded by conjugation, the answer would be that neither is oviparous propagation in the Metazoa invariably preceded by fertilization.