Besides the researches relating to the rule of Mendel and its exceptions, founded, that is, upon a study of the “second” generation of hybrids, there is another important line of research lately inaugurated by Herbst, which investigates the first generation in hybridisation. The hybrids themselves are studied with the special purpose of finding out whether the type of the single hybrid may change according to the conditions of its development, both outer and inner. The discoveries thus made may lead some day to a better understanding of the intimate nature of the “units” concerned in heredity, and perhaps to some knowledge of the arranging and ruling factor in morphogenesis also.
Starting from the discovery of Vernon, that the hybrids of sea-urchins are of different types according to the season, Herbst[132] was able to show that differences among the hybrids with regard to their being more of the paternal or more of the maternal type, are in part certainly due to differences in temperature. But there proved to be still another factor at work, and Herbst has succeeded in discovering this factor by changing the internal conditions of morphogenesis. Whenever he forced the eggs of Sphaerechinus to enter into the first[133] phase of artificial parthenogenesis and then fertilised them with the sperm of Echinus, he was able to approximate the offspring almost completely to the maternal type, whilst under ordinary conditions the hybrids in question follow the paternal far more than the maternal organisation.
What is shown, in the first place, by these discoveries is the importance of an arranging and ruling factor in spite of all units. The organism is always one whole whether the paternal properties prevail or the more complicated maternal ones; in other words, all so-called properties that consist in the spatial relations of parts have nothing to do with “units” or “allelomorphs,” which indeed cannot be more than necessary means or materials, requiring to be ordered. As to the character of the morphogenetic single and separate units themselves Herbst is inclined to regard them as specific chemical substances which unite correspondingly during nuclear conjugation, forming a sort of loose chemical compound. It would depend on the constitution of this compound whether germ-cells of hybrids could become pure or not.
The Rôle of the Nucleus in Inheritance
At the end of our studies on heredity we hardly can avoid saying a few words about the problem of the localisation of the morphogenetic units in the germ-cells themselves. Is it in the protoplasm or in the nucleus that they are placed? You all know that this question was for a long time regarded as more important than any other, and perhaps you have already blamed me for not raising it until now. But in my opinion results gained by the purely analytical method and carefully established, are always superior to those which are of a merely descriptive nature and doubtful besides. The famous problem of the part played by the nucleus in inheritance is both descriptive and doubtful: it is only, so to say, of factual, not of analytical importance, and quite insoluble at present.
As for our second proof of vitalism, stating that no kind of machine inside the germ-cells can possibly be the foundation of their morphogenesis, it is clear that the protoplasm and the nucleus may both come into account here on equal terms. If you prefer to say so, it is to the nucleus and to its division in particular that the second proof of autonomy relates, while the first, though not over-looking the presence of nuclei,[134] deals “especially” with the protoplasmic nature of its “systems.”
What then can we say, on the basis of actual facts, about the part taken by the protoplasm and by the nucleus in inheritance, now that we have learnt from our analytical discussion that both of them cannot be any kind of morphogenetic machine, but can only be means of morphogenesis? Let us state our question in the following way: whereabouts in the germ-cells are those “means” of morphogenesis localised, the existence of which we infer from the material continuity in the course of generations in general and from the facts discovered about hybridisation in particular?
The first of the facts generally said to support the view that the nucleus of the germ-cells exerts a specified influence upon the processes of development and inheritance, relates to the proportion between protoplasm and nuclear material in the egg and in the spermiae. This proportion is very different in the two sexual products, as we know, there being an enormous preponderance of the protoplasm in the egg, of the nucleus in the spermatozoon. This seems to indicate that the proportion between protoplasm and nucleus is fairly indifferent for inheritance, as all the facts go to show that inheritance from the father is as common as inheritance from the mother. It is in the nucleus, and in the nucleus alone, that any similarity of organisation exists between the two sexual products, so very different in all other respects: therefore the nucleus should be the organ of inheritance. The phenomena of nuclear division, of karyokinesis, which are quite equal in both sexual cells, are certainly well fitted to support this hypothesis.
There seems indeed to be some truth in this reasoning, but nevertheless it must remain hypothetical; and it must never be forgotten that there may be very probably some sort of morphogenetic importance in protoplasm also. Rauber and afterwards Boveri[135] have tried to prove experimentally that it is on the nuclear chromatic substance only that inheritance depends, but the first of these authors failed to get any results at all, and the latter obtained only ambiguous ones. Godlewski, on the contrary, has fertilised purely protoplasmic egg-fragments of the sea-urchin with the sperm of quite another group of Echinoderms, and obtained in spite of that a few stages of development of the pure maternal type. This experiment seems to place the morphogenetic importance of protoplasm beyond all doubt.