Plants possess no leucocytes and we therefore see that they tolerate a graft of foreign tissues better than is the case in animals. As a matter of fact heteroplastic grafting is a common practice in horticulture, although even here it is known that indiscriminate heteroplastic grafting is not feasible and that therefore the specificity is not without influence. The host is supposed to furnish only nutritive sap to the graft and in this respect does not behave very differently from an artificial nutritive solution for the raising of a plant. The law of specificity, however, remains true also for the grafted tissues: neither in animals nor in plants does the graft lose its specificity, and it never assumes the specific characters of the host, or _vice versa_. The apparent exceptions which Winkler believed he had found in the case of grafts of nightshade on tomatoes turned out to be a further proof of the law of specificity. Winkler, after the graft had taken, cut through the place of grafting, after which operation a callus formation occurred on the wound. In most cases either a pure nightshade or a pure tomato grew out from this callus. In some cases he obtained shoots from the place where graft and host had united, which on one side were tomato, on the other side nightshade. What really happened was that the shoots had a growing point whose cells on the one side consisted of cells of nightshade, on the other side of tomato.[34] We know of no case in which the cell of a graft has lost its specificity and undergone a transformation into the cell of the host. [34] Baur, E., _Einführung in die experimentelle Vererbungslehre_. Berlin, 1911, p. 232. 3. Another manifestation of the incompatibility of distant species is found in the domain of fertilization. The eggs of the majority of animals cannot develop unless a spermatozoön enters. The entrance of a spermatozoön into an egg seems also to fall under the law of specificity, inasmuch as in general only the sperm of the same or a closely related species is able to enter the egg. The writer[35] has found, however, that it is possible to overcome the limitation of specificity in certain cases by physicochemical means, and by the knowledge of these means we may perhaps one day be able to more closely define the mechanism of specificity in this case. He found that the eggs of a certain Californian sea urchin, which cannot be fertilized by the sperm of starfish in normal sea water, will lose their specificity towards this type of foreign sperm if the sea water is rendered a little more alkaline, or if a little more Ca is added to the sea water, or if both these variations are effected. Godlewski has confirmed the efficiency of this method for the fertilization of sea-urchin eggs with the sperm of crinoids. [35] Literature on this subject in Chapter IV. [Illustration: FIG. 1. Five-days-old larvæ from a sea urchin (Strongylocentrotus purpuratus) ♀ and a starfish (Asterias) ♂. (Front view.)]