I have now described the structure of the chief organs in my oldest larva, and I propose to conclude this paper by a brief summary of the results obtained.
In the first place the mesoblast is not completed ventrally by a layer of cells split off from the hypoblastic yolk-cells, as Scott has described. But the ventral mesoblast is formed by the downgrowth of the mesoblastic plates, which ultimataly meet and unite in the ventral middle line.
The blastopore does not close up, as later observers have maintained, but, as Max Schultze described thirty years ago, it persists as the anus. There is no neurenteric canal, though a solid strand of tissue proceeds back from the alimentary canal and fuses with an indifferentiated mass of cells, into which the nervous system and mesoblast also pass.
The lumen of the alimentary canal is that of the mesenteron; it does not become obliterated during larval life. In its anterior end the hypoblast remains in connection with the epiblast at certain points, and here the gill-clefts arise; between these the mesoblast grows down and forms the gill-bars. The origin of the ciliated ring and the hypopharyngeal groove and hyperpharyngeal bar are also described, and the ciliated condition of the oesophagus and stomach.
The "muscle-plates," whose structure is so peculiar in the Lamprey, arise each from a single cell of the mesoblastic somites. This increases in size, slides in between the neighbouring cells, and ultimately occupies the whole of the space between two myotomes. Its nucleus divides until each cell contains several nuclei. Striated fibrils then appear and increases till the whole "muscle-plate" consists of little else besides these fibrils, squeezing between them a few nuclei. These "muscle-plates" arise from the segmental half of the mesoblast; the muscles of the gills, lips, and probably of the eye, have a different structure and arise from the ventral unsegmented part.
The blood-corpuscles arise from the ventral free edges of the mesoblast, before they unite in the ventral middle line, they collect in a large sinus just behind the heart. The heart appears in the ventral mesentery, formed by the union of the lateral mesoblastic plates; at first its lumen is continuous with the sinus just mentioned. This sinus lies between the hypoblastic yolk-cells and the epiblast; it subsequently acquires walls and forms part of the subintestinal vein.
The ciliated funnels of the pronephros are left as apertures by the segmental duct which in its anterior end is formed from a groove. The groove closes up at intervals, leaving four or five openings which become the funnels. They do not arise as blind projections from the duct, which subsequently, acquire ciliated openings. From the first the pronephros has a double blood supply, pure blood from the aorta passing to the glomerulus, and impure blood in the cardinal veins surrounding the tubuli.
The early development of the skeleton is described up to the stage where Professor Parker commenced his researches.
The canal of the central nervous system developes after the neural chord has separated off from the epidermis; it does not appear to be lined by any invaginated epidermis, as Calberla and Scott maintained.
The first sign of differentiation of the parts of the brain is the formation on the sixteenth day of the optic vesicles and pineal gland. The division into fore-, mid-, and hind-brain appears soon after, but the fore- and mid-brain are not separated by any well-marked groove. The first transverse commissure to appear is situated just in front of the stalk of the pineal gland. It forms the superior commissure of Osborn. Afterwards the ganglion cells thicken round it and form the asymmetrical ganglia habenulæ.
The ganglia on the fifth, seventh, ninth, and tenth nerves are derived from epiblastic thickenings. Their roots probably arise as outgrowths from the neural ridge. The ganglion of the fifth divides into two parts, the ophthalmic and mandibular; these have a common root.
The seventh nerve at its first appearance supplies the first or spiracular gill-cleft; when this is converted into the ciliated ring it continues to be supplied by the seventh nerve.
The connection between the fifth, seventh, and tenth nerve ganglia does not exist and must be of later origin.
The tenth nerve has a large ganglion on its root and bears a ganglion above each of the last six gill-clefts. No trace of the ram us lateralis is to be seen.
The origin of the ganglia on the cranial nerves has no relation to the sense-organs of the skin; these have not appeared even in my oldest larva.