Physiological Relations of Nucleus and Cytoplasm

Boveri ('89, 1) has attempted to test this conclusion by a most ingenious and beautiful experiment ; and although his conclusions do not rest on absolutely certain ground, they at least open the way to a decisive test. The Hertwig brothers showed that the eggs of sea-urchins might be enucleated by shaking, and that spermatozoa would enter the enucleated fragments and cause them to segment. Boveri proved that such fragments would even give rise to dwarf larvae, indistinguishable from the normal in general appearance and differing from the latter only in size and in the very significant fact that their nuclei contain only half the normal number of chromosomes. Now, by fertilizing enucleated egg-fragments of one species (Spherechinus gmnularis) with the spermatozoa of another (Echinus microtuherculatus), Boveri obtained in a few instances dwarf Plutei showing purely paternal characteristics (Fig. 116). From this he concluded that the maternal cytoplasm has no determining effect on the offspring, but supplies only the material in which the spermnucleus operates. Inheritance is, therefore, effected by the nucleus Boveri's result is unfortunately not quite conclusive, as has been pointed out by Seeliger and Morgan, yet his extensive experiments establish, I think, a strong presumption in its favour. Should they be positively confirmed, they would furnish a practical demonstration of inheritance through the nucleus.

The Nucleus in Maturation

Scarcely less convincing, finally, is the contrast between nucleus and cytoplasm in the maturation of the germ-cells. It is scarcely an exaggeration to say that the whole process of maturation, in its broadest sense, renders the cytoplasm of the germ-cells as unlike, the nuclei as like, as possible. The latter undergo a series of complicated changes which are expressly designed to establish a perfect equivalence between them at the time of their union, and, more remotely, a perfect equality of distribution to the embryonic cells. The cytoplasm, on the other hand, undergoes a special and persistent differentiation in each to effect a secondary division of labour between the germ-cells. When this is correlated with the fact that the germ-cells, on the whole, have an equal effect on the specific character of the embryo, we are again forced to the conclusion that this effect must primarily be sought in the nucleus, and that the cytoplasm is in a sense only its agent.