β. THE “MEANS” OF MORPHOGENESIS
We now proceed to an analysis of what may properly be called the means of morphogenesis, the word “means” being preferable to the more usual one “conditions” in this connection, as the latter would not cover the whole field. It is in quite an unpretentious and merely descriptive sense that the expression “means” should be understood at present; what is usually called “conditions” is part of the morphogenetic means in our sense.
β′. The Internal Elementary Means of Morphogenesis
We know that all morphogenesis, typical or atypical, primary or secondary, goes on by one morphogenetic elementary process following the other. Now the very foundation of these elementary processes themselves lies in the elementary functions of the organism as far as they result in the formation of stable visible products. Therefore the elementary functions of the organism may properly be called the internal “means” of morphogenesis.
Secretion and migration are among such functions; the former happening by the aid of chemical change or by physical separation, the latter by the aid of changes in surface tension. But hardly anything more concrete has been made out about these or similar points at present.
We therefore make no claim to offer a complete system of the internal elementary means of morphogenesis. We shall only select from the whole a few topics of remarkable morphogenetic interest, and say a few words about each.
But, first of all, let us observe that the elementary means of morphogenesis are far from being morphogenesis themselves. The word “means” itself implies as much. It would be possible to understand each of these single acts in morphogenesis as well as anything, and yet to be as far from understanding the whole as ever. All means of morphogenesis are only to be considered as the most general frame of events within which morphogenesis occurs.
Some Remarks on the Importance of Surface Tension in Morphogenesis.—There are a few purely physical phenomena which have a special importance in organic morphology, all of them connected with capillarity or surface tension. Soap-lather is a very familiar thing to all of you: you know that the soap-solution is arranged here in very thin planes separated by spaces containing air: it was first proved by Berthold[31] that the arrangement of cells in organic tissues follows the same type as does the arrangement of the single bubbles of a soap-lather, and Bütschli[32] added to this the discovery that the minute structure of the protoplasm itself is that of a foam also. Of course it is not one fluid and one gas which make up the constituents of the structure in the organisms, as is the case in the well-known inorganic foams, but two fluids, which do not mix with one another. One general law holds for all arrangements of this kind: the so-called law of least surfaces, expressed by the words that the sum of all surfaces existing is a minimum; and it again is a consequence of this law, if discussed mathematically, that four lines will always meet in one point and three planes in one line. This feature, together with a certain law about the relation of the angles meeting in one line to the size of the bubbles, is realised most clearly in many structures of organic tissues, and makes it highly probable, at least in some cases, that capillarity is at work here. In other cases, as for instance in many plants, a kind of outside pressure, the so-called tissue tension, may account for the arrangement in surfaces minimae areae. Cleavage stages are perhaps the very best type in which our physical law is expressed: and here it may be said to have quite a simple application whenever all of the blastomeres are of the same physical kind, whilst some complications appear in germs with a specialised organisation and, therefore, with differences in the protoplasm of their single blastomeres. In such instances we may say that the physical law holds as far as the conditions of the system permit, these conditions ordinarily consisting in a sort of non-homogeneity of the surfaces.
It seems, from the researches of Dreyer,[33] that the formation of organic skeletons may also be governed by the physically conditioned arrangement of protoplasmatic or cellular elements, and some phenomena of migration and rearrangement among cleavage cells, as described by Roux, probably also belong here.
But let us never forget that the laws of surface tension only give us the most general type of an arrangement of elements in all these cases, nothing else. A physical law never accounts for the Specific! Capillarity gives us not the least clue to it. As the organic substance, at least in many cases, is a fluid, it must of course follow the general laws of hydrostatics and hydrodynamics, but life itself is as little touched by its fluid-like or foam-like properties as it is by the fact that living bodies have a certain weight and mass.