germ at a period when the type has not yet manifested itself. This led him to the question, "Whether, at the beginning of development, all animals are not essentially alike, and whether a common primordial form does 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

EARLIEST CONDITION OF LARVA.

5 I

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 modifications 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 Cœlenterata, 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 lamine 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

TYPE OF DEVELOPMENT OF ARTICULATA.

53

f

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.

that the Articulata have their navel on their backs. Conversely, it is the characteristic of the evolutionary type of the Vertebrata that the position of the germ corresponds with the dorsal side of the animal. The formation of the dorsal groove, which subsequently closes to form the canal of the spinal cord, as it is gradually enveloped in a sheath growing from below, is followed by the formation of transverse plates, the pre-vertebral plates. The side plates lying outside of these grow towards the ventral side, and finally merge in the navel. The position of the actual vertebral column, consisting of separate vertebræ, is always originally occupied by a cartilaginous band, the notochord (chorda dorsalis), and, as from this axis, the germinal matter transforms itself into a tube above as well as below,―into the spinal marrow with its sheath, and the ventral cavity with the intestinal canal,--Von Baer considered this mode of development as bi-symmetrical. The development of the Articulata he regards as simply symmetrical, and the development of the Molluscs he designated as massive. The justification of this is that the elongation produced by segmentation and the repetition of similar parts and sections of the body implicit in segmentation generally,-the metameric formation, as it is termed by Haeckel,-is totally foreign to the Molluscs.

We must now again repeat, that somewhat extensive. observations of the evolutionary forms of different animals lead at once to the belief that the embryos and evolutionary phases of higher animals are transiently more closely related to the complete and definitive conditions of the lower animal-forms, at least of the same