Co-operation of the determinants to form an organ: insect appendages—Venation of the insect-wing—Deformities in Man—Apex of the fly's leg—Proofs of the existence of determinants—Claws and adhesive lobes—Difference between a theory of development and a theory of heredity—Metamorphosis of the food-canal in insects—Delage's theory—Reinke's theory of the organism-machine—Fechner's views—Apparent contradiction by the facts of developmental mechanics—Formation of the germ-cells—Displacement of the germinal areas in the hydro-medusoid polyps, a proof of the existence of germ-tracks.
It would be futile to attempt to guess at the arrangement of the determinants in the germ-plasm, but so much at least we may say, that the determinants do not lie beside each other in the same disposition as their determinates exhibit in the fully-formed organism. This may be inferred from the complex formative processes of embryogenesis in which many groups of cells, which in their origin were far apart, combine together to form an organ. Thus the arrangement of the determinants in the germ-plasm does not correspond to the subsequent arrangement of the whole animal, nor are primary constituents of the complete organs contained within the germ-plasm. The organ is undoubtedly predetermined in the germ-plasm, but it is not preformed as such.
Here, again, the history of development gives us a certain basis of fact from which to work. Let us consider, for instance, the origin of the appendages in those insects which in the larval state possess neither legs nor wings, but exhibit a gradual emergence of these structures from concealment underneath the integumentary skeleton. In these cases, as I have already shown in regard to the wings, the development of the limbs arises from definite groups of cells in the skin. These must therefore be regarded as the formative, and therefore as the most important and indispensable, parts of the rudiments, and may be designated the imaginal disks, as I many years ago proposed[22] (Fig. 89, ui and oi).
[22] Die Entwicklung der Dipteren, Leipzig, 1864.
But these disks of cells do not contain the whole leg, but only the skin-layer of it, the 'hypodermis,' which, however, in this case undoubtedly determines the form. But the internal parts of the leg, especially the nerves, tracheæ, and probably also the muscles, are formed from other cell-groups and grow into the imaginal disk from outside. Something similar probably takes place in the case of all organs which are made up of many parts; they are, so to speak, shot together from several points of origin, from various primordia; and determinants are brought into co-operation whose relative value in determining the form and function of the organ may be very diverse.
Fig. 89. Anterior region of the larva
of a Midge (Corethra plumicornis). K, head.
Th, thorax. ui, inferior imaginal disks.
oi, superior imaginal disks. ui1, ui2, and
ui3, the primordia of the limbs. oi2
and oi3, the primordia of the wings and
'balancers.' g, brain. bg, chain of ventral
ganglia with nerves which enter the
imaginal disks. trb, tracheal vesicle.
Enlarged about 15 times.
For it is undoubtedly a very different matter whether a cell bears within it the elements which compel it in the course of growth to develop an organ, for instance a leg, of quite definite size, sculpture, number of joints, and so on, or whether it only bears the somewhat vague power of determining that connective tissue or fatty tissue is to be produced. In the first case it controls the whole formation of the part, in the second it only fills up gaps or lays down fat or other substances within itself if these be presented to it. Between these two extremes of determining power there are many intermediate stages. Cells which contain the determinants of blood-vessels, tracheæ, or nerves need not be so definitely determined that they always give rise to precisely the same blood-vessels, the same branching of the tracheæ, or the same bifurcation of nerves; they may probably possess no more than the general tendency to the formation of such parts, and the special form taken by the nerves, tracheæ, or blood-vessels may be essentially determined by their environment. Thus in the morbid tumours of Man, nerves, and especially blood-vessels, may develop in a quite characteristic manner, which was certainly not determined in advance, but has been called forth by the stimulus, the pressure, and other influences of the cellular basis of the tumour. In short, the cells were only determined to this extent, that they contained the tendency to give rise to blood-vessels under particular influences.
It would be a mistake, however, to think of the primary constituents of all cell-groups as so indefinite. Let us call to mind, for instance, the venation of the insect wing. It is well known that this is not only quite different in beetles, bugs, and Diptera from that in the Hymenoptera, and different again in the butterflies, but that it is quite characteristic in every individual family of butterflies, and indeed in every genus. We cannot conceive of the absolute certainty of development of these very characteristic and constant branchings as having its roots elsewhere than in the determinants of the germ-plasm, which, lying within certain series of cells, ultimately cause particular cell-series of the wing-rudiment to become the wing-veins. If this were not so, how would it be possible to understand the fact that every minute deviation in the course of these veins is repeated in exactly the same way in all the individuals of a genus, while in all the individuals of an allied genus the venation turns out slightly different with equal constancy.
But it is quite certain that all determinations are in some degree susceptible to modifying influences, that they are in very different degrees capable of variation.