But Delage has shown that a non-nucleated fragment of the ovum in some of the lower animals, as, for example, the sea-urchin, can give rise to a daughter organism with the normal number of chromosomes when fertilised by a spermatozoon. Conversely, Loeb showed that the nucleus of the spermatozoon can be dispensed with. Thus it seems that either the egg or the spermatozoon of the sea-urchin contains all the essential elements for the production of the perfect larva of a daughter organism. We are, therefore, driven to the conclusion that the fertilised ovum contains two sets of fully-equipped units. Only one of these seems to contribute to the developing organism. If this set happens to be composed of material derived from one only of the parents, we can see how it is that we get unilateral inheritance in the case of a cross. Where, however, the units from the two parents intermingle, although only one set is active in development, the result will be blended inheritance. Thus, we may regard the fertilised egg as made up of two sets of characters—a dominant set, which is active in the production of the resulting organism, and a recessive set, which appears to take little or no part in the production of the organism. This is quite in accordance with Mendelian conceptions. Let X be an organism having the unit characters A _B_ C D _E_ F _G_, and let Y be another organism having the unit characters _a_ b _c_ _d_ e _f_ g. Now suppose that these behave as opposed Mendelian units, and that the unit characters in italics are dominant ones. Then the resulting individual will resemble each parent in certain unit characters. It may be represented by the formula a B c d E f G, but it will contain the characters A b C D e F g in a recessive form, so that its complete formula may be written a B c d E f G} A b C D e F g When these hybrids are paired together it will be _possible_ to get such forms as A B C D E F G A B C D E F G and a b c d e f g a b c d e f g which exactly resemble the respective grandparents, and these should breed absolutely true, if the segregation of the gametes is as pure as the Mendel’s law seems to require. Experiments of Cuénot and Castle