Quite in harmony with this hypothesis concerning the respective natures of the sperm-cell and germ-cell is a remark of Carpenter's on the same point:—

"Looking," he says, "to the very equal mode in which the characters of the two parents are mingled in hybrid offspring, and to the certainty that the material conditions which determine the development of the germ are almost exclusively female, it would seem probable that the dynamical conditions are, in great part, furnished by the male."[[94]]

§ 12. Could nothing but the foregoing indirect evidence be adduced in proof of the proposition that the spermatozoon is essentially a neural element, and the ovum essentially a hæmal element, we should scarcely claim for it anything more than plausibility. On finding, however, that this indirect evidence is merely introductory to evidence of a quite direct nature, its significance will become apparent. Adding to their weight taken separately the force of their mutual confirmation, these two series of proofs will be seen to give the hypothesis a high degree of probability. The direct evidence now to be considered is of several kinds.

On referring to the description of the process of multiplication in monads, quoted some pages back ([§ 5]), from Professor Owen, the reader will perceive that it is by the pellucid nucleus that the growth and reproduction of these single-celled creatures are regulated. The nucleus controls the circulation of the plasmatic fluid; the fission of the nucleus is the first step towards the formation of another cell; each half of the divided nucleus establishes round itself an independent current; and, apparently, it is by the repulsion of the nuclei that the separation into two individuals is finally effected. All which facts, when generalised, imply that the nucleus is the governing or co-ordinating part. Now, Professor Owen subsequently points out that the matter of the sperm-cell performs in the fertilised germ-cell just this same function which the nucleus performs in a single-celled animal. We find the absorption by a germ-cell of the contents of a sperm-cell "followed by the appearance of a pellucid nucleus in the centre of the opaque and altered germ-cell; we further see its successive fissions governed by the preliminary division of the pellucid centre;" and, led by these and other facts, Professor Owen thinks that "one cannot reasonably suppose that the nature and properties of the nucleus of the impregnated germ-cell and that of the monad can be different."[[95]] And hence he further infers that "the nucleus of the monad is of a nature similar to, if not identical with," the matter of the spermatozoon. But we have seen that in the monad the nucleus is the co-ordinating part; and hence to say that the sperm-cell is, in nature, identical with it, is to say that the sperm-cell consists of co-ordinating matter.

Chemical analysis affords further evidence, though, from the imperfect data at present obtained, less conclusive evidence than could be wished. Partly from the white and gray nervous substances having been analysed together instead of separately, and partly from the difficulty of isolating the efficient contents of the sperm-cells, a satisfactory comparison cannot be made. Nevertheless, possessing in common, as they do, one element, by which they are specially characterised, the analysis, as far as it goes, supports our argument. The following table, which has been made up from data given in the Cyclopædia of Anatomy and Physiology, Art. Nervous System, gives the proportion of this element in the brain in different conditions, and shows how important is its presence.

In
Infants.		In
Youth.		In
Adults.		In
Old Men.		In
Idiots.
Solid constituents in a hundred
parts of the brain		17.2125.7427.4926.1529.07
Of these solid constituents the
phosphorus amounts to		0.81.651.801.000.85
Which gives a percentage of
phosphorus in the solid
constituents of		4.656.416.543.822.92

This connection between the quantity of phosphorus present and the degree of mental power exhibited, is sufficiently significant; and the fact that in the same individual the varying degrees of cerebral activity are indicated by the varying quantities of alkaline phosphates excreted by the kidneys,[[96]] still more clearly shows the essentialness of phosphorus as a constituent of nervous matter. Respecting the constitution of sperm-cells chemists do not altogether agree. One thing, however, is certain—that they contain unoxidized phosphorus; and also a fatty acid, that is not improbably similar to the fatty acid contained in neurine.[[97]] In fact, there would seem to be present the constituents of that oleophosphoric acid which forms so distinctive an element of the brain. That a large quantity of binoxide of protein is also present, may be ascribed to the fact that a great part of the sperm-cell consists merely of the protective membrane and its locomotive appendage; the really efficient portion being but the central contents.[[98]]

Evidence of a more conclusive nature—evidence, too, which will show in what direction our argument tends—is seen in the marked antagonism of the nervous and generative systems. Thus, the fact that intense mental application, involving great waste of the nervous tissues, and a corresponding consumption of nervous matter for their repair, is accompanied by a cessation in the production of sperm-cells, gives strong support to the hypothesis that the sperm-cells consist essentially of neurine. And this becomes yet clearer on finding that the converse fact is true—that undue production of sperm-cells involves cerebral inactivity. The first result of a morbid excess in this direction is headache, which may be taken to indicate that the brain is out of repair; this is followed by stupidity; should the disorder continue, imbecility supervenes, ending occasionally in insanity.

That the sperm-cell is co-ordinating matter, and the germ-cell matter to be co-ordinated, is, therefore, an hypothesis not only having much à priori probability, but one supported by numerous facts.

§ 13. This hypothesis alike explains, and is confirmed by, the truth, that throughout the vertebrate tribes the degree of fertility varies inversely as the development of the nervous system.