and in a certain sense, by a particular gradual evolution of the organs, a bird also, but made the embryo likewise repeat and surpass the lower types. To this false tendency, acting on vague analogies, a stop was put by the great naturalist just named. He showed that a number of coincidences might, indeed, be demonstrated between the embryo of the higher and the permanent form of the lower animals, but that this resemblance rested essentially on the fact that in the embryo of the higher animal the differentiation of the general fundamental mass had not yet set in, and that in the progress of development it passes through stages which are permanent in the series of inferior animals.

On the other hand, he positively repudiated the assertion that the embryos of the higher types actually pass through forms permanent in the lower ones. He says that the type of each animal seems from the first to fix itself in the embryo, and to regulate its whole development. As regards the vertebrate animals in particular, the further we go back in the history of their development, the more do we find the embryos alike, both on the whole and in the individual parts. "Only gradually do the characters appear which mark the greater, and later those which mark the smaller divisions of the Vertebrata. Thus from the general type the special one is evolved."

Von Baer thus held that the analogy consisted only in the embryonic states of the various animal forms; but he was obliged to go beyond the circle of the types, and he thought it probable that among all embryos of vertebrate, as well as invertebrate animals, developed from a true ovum, there is a conformity in the condition of the

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THE DOCTRINE OF DESCENT.

germat a period when the type has not yet manifested self. This led him to the question, "Whether, at the beginning of development, all animals are not essentrally ake, and whether a common primordial form oes not exist for all?" "It might,” he finally thinks, be maintained, not without reason, that the simple cyst-like form is the common fundamental form from which all animals are developed, not merely in idea, but historically."

When the barrier which it was formerly thought necessary to erect between asexual multiplication and multiplication caused by fecundation had been recognized as non-existent, and it was perceived that all development amounts to the multiplication and metamorphosis of the primitive germ or egg-cell, the cell was necessarily regarded, in the acceptation of the older investigators, as the common fundamental form. But although the descriptive history of evolution does not go back to this elementary organism, and considers even the bifurcation as merely a preparation for actual development, at any rate the earliest rudimentary larval conditions of different types may be compared with each other.

The discoveries of the last ten years with reference to this subject are so numerous, and such striking analogies have been advanced, that we must needs go much further than, at that time, was possible for Von Baer. It is not merely a question of those general analogies in the segregation of tissues from an indifferent rudimentary mass, but of homologies in the distribution, form, and composition of the embryos and larvæ, of which the after effects are of profound importance

to the later and actual typical impress. With this object, let us consider the larva of a calcareous sponge at the stage which Haeckel has designated as the Gastrula phase.

The diagram gives the section of a larva of this description, which at this period is nothing more than a stomach provided with an orifice (fig. 5 0); its wall con

sists of two strata, or layers of cells. The cells of the external stratum are distinguished from those of the inner one by their elongated form, and the possession of filaments serving as organs of locomotion. All subsequent development and differentiation, certainly not very important in the sponges, may be traced to modi-* fications of these two membranes; the external mem

brane (Ectoderm, or Exoderm) and the internal membrane (Entoderm). And this phase of the ciliated larva, with its twofold strata, its primitive ventral cavity and mouth, recurs in the Coelenterata, with slight variations in the Echinoderms, in some of the Annulosa, in the Sagitta, the Ascidians, and the Lancelet. From the analogy of all these animals, and especially of the last, we shall be able hereafter to derive important inductions.

But if no weight be attached to the presence of these filaments of the external layer, which is, moreover, justified by the relation of the filament to the cell, and if it be acknowledged as the essential significance of the larval arrangement, that from its two laminæ the collective organs derive their origin, then to the animals above enumerated must be added, not only almost the whole of the Articulata, but likewise the remainder of the Vertebrata, as in them, immediately after the appearance of the primitive striæ, follows their separation into two cell-layers, or membranes. Respecting the derivation of the third or middle germinal lamina, and the share of the two primitive laminæ in its formation, observers are not agreed.

Only from this point does the development of the great animal groups take various directions, and it is the immortal merit of Von Baer to have fixed these types of development, independently of the fundamental forms, established by Cuvier on zoological and anatomical considerations, and he thereby laid a far deeper foundation for the existence of these types. We will illustrate our meaning by two examples.

When the ovum of the articulate animal has sur

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ITL

rounded itself with a germinal membrane, a portion of it thickens into a long germinal stria, resembling an elongated ellipse. This is the rudiment of the ventral side of the future animal. A groove then divides it into the two germinal laminæ, and transverse striæ next make their appearance, the indications of the so-called primordial segments. The symmetrical disposition of the organs, and the integration of the body out of consecutive segments, is herewith initiated. All further development 'emanates from these primordial segments, which are the standard of the Annelids or higher Vermes; while in the Articulata, projections and appendages of these segments develop into feelers, manducatory apparatus and legs, and by their heterogeneous integration in the regions of the head, and of the middle and posterior portions of the body, give rise to the vast variety within the type. In each particular case we see what is special emanate from what is more homogeneous and undifferentiated, and this is likewise corroborated by the more advanced phase portrayed in the diagram (fig. 6). It represents the embryo of the great black-beetle (Hydrophilus piceus) on its ventral side. The antennæ (ƒ), the three pair of oral appendages (m), and the three pair of legs, are as yet little distinguished. In the further course of development, the lateral portions grow towards the back, in the centre of which they finally meet. As compared with the Vertebrata, it may hence be said

FIG. 6.