This point is so important that I must illustrate it by a few more examples. The so-called polar bodies (to be treated more in detail below) which are expelled during maturation from the eggs of so many animals, are true cells, as was first proved by Bütschli in Nematodes: their formation is due to a process of undoubted cell-division usually accompanied by a typical form of indirect nuclear division[121]. If any one is still in doubt upon this point, after the observations of Fol and Hertwig, he might easily be convinced of its truth by a glance at the figures (unfortunately too little known) which Trinchese[122] has published, illustrating this process in the eggs of certain gastropods. The eggs of _Amphorina coerulea_ are in every way suitable for observation, being entirely translucent, and having large distinct nuclei which differ from the green cytoplasm in colour. In these eggs two polar bodies are formed one after the other: and each of them immediately re-divides: hence it follows that four polar bodies are placed at the pole of the egg. But how is it that these four cells perish, while the nucleus, remaining in the yolk and conjugating with the sperm-nucleus, makes use of the whole body of the egg and developes into the embryo? Obviously because the nature of the polar body is different from that of the egg-cell. But since the nature of the cell is determined by the quality of the nucleus, this quality must differ from the very moment of nuclear division. This is proved by the fact that the supernumerary spermatozoa which sometimes enter the egg do not conjugate with the polar bodies. According to Strasburger’s theory, the objection might be urged that the different quality of the nuclei is here caused by the very different quantity of cytoplasm by which they are surrounded and nourished; but on the one hand the smallness of the cell-bodies which surround most polar globules must have some explanation, and this can only be found in the nature of the nucleus; and on the other hand the quantity of the cell-body which surrounds the polar globules of _Amphorina_ is, as a matter of fact, somewhat larger than the sphere of green cytoplasm which surrounds the nucleus of the egg! The difference between the polar bodies and the egg-cell can thus only be explained on the supposition that, in the division of the nuclear spindle, two qualitatively different daughter-nuclei are produced. There does not seem to be any objection to the view that the microsomata of the nuclear loops—assuming that these bodies represent the idioplasm—are capable of dividing into halves, equal in form and appearance, but unequal in quality. We know that this very process takes place in many egg-cells; thus in the egg of the earth-worm the first two segmentation spheres are equal in size and appearance, and yet the one forms the endoderm and the other the ectoderm of the embryo.