Fig. 94.Diagram to illustrate the phylogenetic shifting back of the origins of the germ-cells in medusoids and hydroids. A composite picture.A, branch of a polyp colony.P, polyp-head with mouth (m) and tentacles.St, stalk of the polyp.M, medusoid-bud with the bell (Gl).T, marginal tentacle.m, mouth.Mst, manubrium.GphK, a gonophore-bud.GH, gastric cavity.ekt, ectoderm.ent, endoderm.st, supporting lamella. The germ-cells (kz) arise in the medusoid in the ectoderm of the manubrium—first phyletic stage—where they also attain maturity. In the gonophore-bud (GphK) they arise in the ectoderm (kz´), or further down in the stalk of the polyp atkz´´—third phyletic stage, or in the ectoderm of the branch from which the polyp has arisen, atkz´´´—fourth phyletic stage of the shunting of the originative area of the germ-cells. In the two last cases the germ-cells migrate until they reach their primitive place of origination in the medusoid, or in the corresponding layer of the medusoid gonophore, as may be more clearly seen in Fig. 95. Drawn from my sketch by Dr. Petrunkewitsch.
Fig. 94.Diagram to illustrate the phylogenetic shifting back of the origins of the germ-cells in medusoids and hydroids. A composite picture.A, branch of a polyp colony.P, polyp-head with mouth (m) and tentacles.St, stalk of the polyp.M, medusoid-bud with the bell (Gl).T, marginal tentacle.m, mouth.Mst, manubrium.GphK, a gonophore-bud.GH, gastric cavity.ekt, ectoderm.ent, endoderm.st, supporting lamella. The germ-cells (kz) arise in the medusoid in the ectoderm of the manubrium—first phyletic stage—where they also attain maturity. In the gonophore-bud (GphK) they arise in the ectoderm (kz´), or further down in the stalk of the polyp atkz´´—third phyletic stage, or in the ectoderm of the branch from which the polyp has arisen, atkz´´´—fourth phyletic stage of the shunting of the originative area of the germ-cells. In the two last cases the germ-cells migrate until they reach their primitive place of origination in the medusoid, or in the corresponding layer of the medusoid gonophore, as may be more clearly seen in Fig. 95. Drawn from my sketch by Dr. Petrunkewitsch.
In the hydroid polyps and their medusoids the germ-cells always arise in the ectoderm; in species which produce sexual medusoids by budding, the germ-cells arise in the ectoderm of the manubrium of these medusoids (Fig. 94,M,kz). But in many species these sexual stages have degenerated in the course of phylogeny into so-called gonophores, that is, to medusoids which still exhibit more or less complete bells, but neither mouth (m) nor marginal tentacles (T), and which no longer break away from the colony to swim freely about, to feed independently, and to produce and ripen germ-cells. The degeneration of the 'gonophores' often goes even further; in many the medusoid bell is represented only by a thin layer of cells, and in some even this token of descent from medusoid ancestry is absent, and they are mere single-layered closed brood-sacs (Fig. 95,Gph).
The adherence of the sexual animal to the hydroid colony has, however, made a more rapid ripening of the germ-cells possible, and nature has taken advantage of this possibility in all the cases known to me, for the germ-cells no longer arise in the manubrium of the mature degenerate medusoid, that is, of the gonophore, butearlier, before the bud which becomes a gonophore possesses a manubrium. The birthplace of the germ-cells is thus shifted back from the manubrium of the medusoid to the young gonophore-bud (Fig. 94,M,kz). The same thing occurs in species in which the medusoids are liberated, but live only for a short time, for instance, in the genusPodocoryne. Although perfect medusoids are formed, these have their germ-cells fully developed at the time of their liberation from the hydroid colony. But in species in which the medusoid-buds have really degenerated and are no longer liberated, the birthplace of the germ-cells is shiftedeven further back, and in the first place into the stalk (St,kz´´) of the polyp from the gonophore-buds. This is the case in the genusHydractinia. In the further course of the process the birthplace of the germ-cells has shifted as far back as to the branch from which the polyp has grown out (Fig. 94,A,kz´´´); and finally, in the cases in which the medusoid has degenerated to a mere brood-sac (Fig. 95,Gph), even to the generation of polyps immediately before, that is, into the polyp-stem from which the branch arises that bears the polyps producing the gonophore-bud (Fig. 95,kz´´´). Then we find the birthplace of the germ-cellsstillfurther back (Fig. 95,kz´´´´), for the egg and sperm-cells arise in the stem of the principal polyps (the main stem of the colony). The advantage of this arrangementis easily seen, for the principal polyp is present earlier than those of the secondary branches, and these again earlier than the polyp which bears the sexual buds, and this, finally, earlier than the sexual bud which it bears. Thus this shunting backwards of the birthplace of the germ-cells means an earlier origin of the primordium (Anlage) of the germ-cells, and consequently an earlier maturing of these.
Fig. 95.Diagram to illustrate the migration of the germ-cells in hydro-medusæ from their remotely shunted place of origin to their primitive place of origin in the gonophore, in which they attain to maturity. The state of affairs in Eudendrium is taken as the basis of the diagram.HP, one of the principal polyps.mu, mouth.ma, gut-cavity.t, tentacle.Sta, its stem.A, a branch of the polyp colony.SP, lateral polyp.Gph, a medusoid-bud completely degenerated into a mere gonophore.Ei, ovum.GH, gastric cavity.st, supporting lamella. The originative area of the germ-cells lies in the stem of the principal polyp atkz´´´´, whence the germ-cells first migrate into the endoderm of the branch (A) atkz´´´, creeping within which they reachkz´´in the lateral polyp (blastostyle), finally reaching the gonophore (kz) and passing again into the ectoderm. Drawn from my sketch by Dr. Petrunkewitsch.
Fig. 95.Diagram to illustrate the migration of the germ-cells in hydro-medusæ from their remotely shunted place of origin to their primitive place of origin in the gonophore, in which they attain to maturity. The state of affairs in Eudendrium is taken as the basis of the diagram.HP, one of the principal polyps.mu, mouth.ma, gut-cavity.t, tentacle.Sta, its stem.A, a branch of the polyp colony.SP, lateral polyp.Gph, a medusoid-bud completely degenerated into a mere gonophore.Ei, ovum.GH, gastric cavity.st, supporting lamella. The originative area of the germ-cells lies in the stem of the principal polyp atkz´´´´, whence the germ-cells first migrate into the endoderm of the branch (A) atkz´´´, creeping within which they reachkz´´in the lateral polyp (blastostyle), finally reaching the gonophore (kz) and passing again into the ectoderm. Drawn from my sketch by Dr. Petrunkewitsch.
But none of all these germ-cells come to maturity in the birthplace to which they have been shifted, for they migrate independently from it to the place at which they primitively arose, namely, into themanubrium of the medusoid, which is still present even when great degeneration has occurred, or even—in the most extreme cases of degeneration—into the ectoderm of the brood-sac. This is the case in the genusEudendrium, of which Fig. 95 gives a diagrammatic representation.
The most interesting feature of this migration of the germ-cells is that the cells invariably arise in the ectoderm (kz´´´´), then pierce through the supporting lamella (st) into the endoderm (kz´´´), and then creep along it to their maturing-place. Once there they break through again to the outer layer of cells, the ectoderm (kz), and come to maturity (Ei). That they make their way through the endoderm is probably to be explained by the fact that they are there in direct proximity to the food-stream which flows through the colony (GH= gastric cavity), and they are thus more richly nourished there than in the ectoderm. But although this is the case, they never arise in the endoderm; in no single case is the birthplace of the germ-cells to be found in the endoderm, but always in the ectoderm, no matter how far back it may have been shunted. Even when the germ-cells migrate through the endoderm, their first recognizable appearance is invariably in the ectoderm, as, for instance, inPodocoryneandHydractinia. The course of affairs is thus exactly what it would necessarily be if our supposition were correct, that only definite cell-generations—in this case the ectoderm-cells—contain the complete germ-plasm. If the endoderm-cells also contained germ-plasm it would be hard to understand why the germ-cells never arise from them, since their situation offers much better conditions for their further development than that of the ectoderm-cells. It would also be hard to understand why such a circuitous route was chosen as that exhibited by the migration of the young germ-cells into the endoderm. Something must be lacking in the endoderm that is necessary to make a cell into a germ-cell: that something is the germ-plasm.
If we accept the theory of the continuity of the germ-plasm as in the main correct, it appears that higher animals and plants are constructed of two kinds of elements, the somatic cells and the germ-cells; both owe their being to the germ-plasm of the ovum, but the former do not contain it complete but only in individual determinants[24],and therefore can never give rise again to the rank of germ-cells; the others contain the latent germ-plasm intact, and can therefore produce not only cells like themselves for a certain time by division, but have also the power, when they are mature and the necessary conditions have been fulfilled, of bringing forth a new individual of the same species. The former have only a limited length of life, they die—they must necessarily die—when the life of the individual to which they belong is at an end; the latter are potentially immortal, like the unicellular organisms, that is, they can in favourable circumstances give rise to the germ-cells of a new individual, and so on for all time, as far as we can see. The germ-plasm of a species is thus never formedde novo, but it grows and increases ceaselessly; it is handed on from one generation to another like a long root creeping through the earth, from which at regular distances shoots grow up and become plants, the individuals of the successive generations. If these conditions be considered from the point of view of reproduction, the germ-cells appear the most important part of the individual, for they alone maintain the species, and the body sinks down almost to the level of a mere cradle for the germ-cells, a place in which they are formed, and under favourable conditions are nourished, multiply, and attain to maturity. But the matter can also be looked at in an opposite light, and then the endless root of the germ-plasm, with its germ-cells ever forming new individuals, may be regarded as the means by which alone nature was able to create multicellular organisms, individuals of higher and higher differentiation and capacity, able to adapt themselves to all possible conditions, and to make the fullest use of all the possibilities of life.
[24]Boveri has recently made an observation upon the thread-worm of the horse, which points to the correctness of the conception of the germ-plasm. The two first segmentation-cells both receive the four chromosomes of the species, but, in one of the two, a portion of the chromatin breaks off and degenerates, or dissolves, at least as far as can be seen. The other cell retains the whole mass of chromatin, and from this there arise later the primitive genital-cells. In the germ-track, therefore—so we must interpret it—the whole of the germ-plasm is retained, while a part of it is withdrawn from the soma. I have only partly described the process, and I do not wish to enter in detail on an interpretation of it, since it seems to me obscure and to require further observations before an interpretation can be attempted with any confidence.
[24]Boveri has recently made an observation upon the thread-worm of the horse, which points to the correctness of the conception of the germ-plasm. The two first segmentation-cells both receive the four chromosomes of the species, but, in one of the two, a portion of the chromatin breaks off and degenerates, or dissolves, at least as far as can be seen. The other cell retains the whole mass of chromatin, and from this there arise later the primitive genital-cells. In the germ-track, therefore—so we must interpret it—the whole of the germ-plasm is retained, while a part of it is withdrawn from the soma. I have only partly described the process, and I do not wish to enter in detail on an interpretation of it, since it seems to me obscure and to require further observations before an interpretation can be attempted with any confidence.