[A]An interesting problem concerning the atmosphere is suggested by certain geological facts. In our buried coal-seams and other carbonaceous deposits a great quantity of carbon, for the most part abstracted from the atmosphere, has been stored away. Still greater quantities of carbon are imprisoned in the substance of our limestones, which contain, when pure, 44 per cent. of this element. A large quantity of oxygen has also been taken from the atmosphere to combine with other elements during their oxidation. The question is—Was the atmosphere, in the geological past, more richly laden with carbonic acid gas, of which some has entered into combination with lime to form limestone, while some has been decomposed by plants, the carbon being buried as coal, and the oxygen as products of oxidation? Or, has the atmosphere been furnished with continuous fresh supplies of carbonic acid gas?
[B]It has before been noticed that the organs themselves have their periods of rest. The rhythm of rest and repose in the heart is not that of the activity and sleep of the organism, but that of the contraction and relaxation of the organ itself.
[C]From a popular article of the author's on "Horns and Antlers," in Atalanta.
[D]It will be well here to introduce the technical terms for these changes. The general term for chemical actions occurring in the tissues of a living creature is metabolism; where the change is of such a nature that complex and unstable compounds are built up and stored for a while, it is called anabolism; where complex unstable compounds break up into less complex and relatively stable compounds, the term katabolism is applied. We shall speak of anabolic changes as constructive; katabolic, as disruptive, or sometimes, explosive.
[E]I do not mean, of course, to imply that there is no reconstruction during activity, but that it is then distinctly outbalanced by disruptive changes.
[F]Professor Geddes and Mr. J. Arthur Thomson, in their interesting work on "The Evolution of Sex," regard the ovum in especial, and the female in general, as preponderatingly anabolic (see note, [p. 32]); while the sperm in especial, and the male in general, are on their view preponderatingly katabolic. Regarding, as I do, the food-yolk as a katabolic product, I cannot altogether follow them. The differentiation seems to me to have taken place along divergent lines of katabolism. In the ovum, katabolism has given rise to storage products; in the sperm, to motor activities associated with a tendency to fission. The contrast is not between anabolic and katabolic tendencies, but between storage katabolism and motor katabolism. Nor do I think that "the essentially katabolic male-cell brings to the ovum a supply of characteristic waste products, or katastates, which stimulate the latter to division" (l.c., [p. 162]). I believe that it brings an inherited tendency to fission, and thus reintroduces into the fertilized ovum the tendency which, as ovum, it had renounced in favour of storage katabolism.
[G]On the other hand, three ova of the crustacean Apus are said to coalesce to form the single ovum from which one embryo develops.
[H]"The Evolution of Sex," [p. 84].
[I]In some forms of life the opening of the cup marks the position of the future mouth: in others, of the future vent. In yet others it elongates into a slit, occupying the whole length of the embryo; the middle part of the slit closes up, and the opening at the far ends mark the position, the one of the future mouth, the other of the future vent.
[J]In technical language, the outer layer of cells is called the epiblast, the inner layer the hypoblast, and the mid-layer between them the mesoblast.