At a certain stage in the segmentation the cells of the ovum become divided into two layers, an epiblast investing the whole of the ovum with the exception of a small area adjoining the placenta, where the inner layer or hypoblast, which forms the main mass of the ovum, projects at the surface. The epiblast soon covers the whole of the hypoblast, so that there would seem (according to Salensky’s observations) to be a kind of epibolic invagination: a conclusion supported by Todaro’s figures.
At a later stage, on one side of the free apex of the embryo, a mesoblastic layer makes its appearance between the epiblast and hypoblast. This layer is derived by Salensky, as it appears to me on insufficient grounds, from the epiblast. Nearly at the same time there arises not far from the same point of the embryo, but on the opposite side, a solid thickening of epiblast which forms the rudiment of the nervous system. The nervous system is placed close to the front end of the body; and nearly at the opposite pole, and therefore at the hind end, there appears immediately below the epiblast a mass of cells forming a provisional organ known as the elæoblast. Todaro regards this organ as mesoblastic in origin, and Salensky as hypoblastic. The organ is situated in the position which would be occupied by the larval tail were it developed. It may probably be regarded (Salensky) as a disappearing rudiment of the tail, and be compared in this respect with the more or less similar mass of cells described by Kupffer in Molgula, and with the elæoblast in Pyrosoma.
After the differentiation of these organs a cavity makes its appearance between the epiblast and hypoblast, which is regarded by Salensky as the body cavity. It appears to be equivalent to the segmentation cavity of Todaro. According to Todaro’s statements, it is replaced by a second cavity, which appears between the splanchnic and somatic layers of mesoblast, and constitutes the true body cavity. The embryo now begins to elongate, and at the same time a cavity makes its appearance in the centre of the hypoblast cells. This cavity is the rudiment of the branchial and alimentary cavities: on its dorsal wall is a median projection, the rudiment of the so-called gill of Salpa.
At two points this cavity comes into close contact with the external skin. At one of these, situated immediately ventral to the nervous system, the mouth becomes formed at a later period. At the other, placed on the dorsal surface between the nervous system and the elæoblast, is formed the cloacal aperture.
By the stage under consideration the more important systems of organs are established, and the remaining embryonic history may be very briefly narrated.
The embryo at this stage is no longer covered by the walls of the brood-pouch but projects freely into the atrial cavity, and is only attached to its parent by means of the placenta. The epiblast cells soon give rise to a deposit which forms the mantle. The deposit appears however to be formed not only on the outer side of the epiblast but also on the inner side; so that the epiblast becomes cemented to the subjacent parts, branchial sack, etc., by an intercellular layer, which would seem to fill up the primitive body cavity with the exception of the vascular channels (Salensky).
The nervous system, after its separation from the epiblast, acquires a central cavity, and subsequently becomes divided into three lobes, each with an internal protuberance. At its anterior extremity it opens into the branchial sack; and from this part is developed the ciliated pit of the adult. The nervous ganglion at a later period becomes solid, and a median eye is subsequently formed as an outgrowth from it.
According to Todaro there are further formed two small auditory (? olfactory) sacks on the ventral surface of the brain, each of them placed in communication with the branchial cavity by a narrow canal.
The mesoblast gives rise to the muscles of the branchial sack, to the heart, and to the pericardium. The two latter are situated on the ventral side of the posterior extremity of the branchial cavity.
Branchial sack and alimentary tract. The first development of the enteric cavity has already been described. The true alimentary tract is formed as a bud from the hinder end of the primitive cavity. The remainder of the primitive cavity gives rise to the branchial sack. The so-called gill has at first the form of a lamella attached dorsally to the walls of the branchial sack; but its attachment becomes severed except at the two ends, and it then forms a band stretching obliquely across the branchial cavity, which subsequently becomes hollow and filled with blood corpuscles. The whole structure is probably homologous with the peculiar fold, usually prolonged into numerous processes, which normally projects from the dorsal wall of the Ascidian branchial sack.