The formation of the central nervous system commences with the flattening of the dorsal surface of the embryo. The flattened area forms a plate ([fig. 2] B and [fig. 3] A, np), extending backwards to the blastopore, which has in the meantime passed round to the dorsal surface. The sides of the plate become raised as two folds, which are most prominent posteriorly, and meet behind the blastopore, but shade off in front. The two folds next unite dorsally, so as to convert the previous groove into a canal[3]—the neural or medullary canal. They unite first of all over the blastopore, and their line of junction extends from this point forwards ([fig. 2] C, D, E). There is in this way formed a tube on the floor of which the blastopore opens behind, and which is itself open in front. Finally the medullary canal is formed for the whole length of the embryo. The anterior opening persists however for some time. The communication between the neural and alimentary tracts becomes interrupted when the caudal fin appears and the anus is formed. The neural canal then extends round the end of the notochord to the ventral side, but subsequently retreats to the dorsal side and terminates in a slight dilatation.
In the formation of the medullary canal there are two points deserving notice—viz. (1) the connection with the blastopore; (2) the relation of the walls of the canal to the adjoining epiblast. With reference to the first of these points it is clear that the fact of the blastopore opening on the floor of the neural canal causes a free communication to exist between the archenteron or gastrula cavity and the neural canal; and that, so long as the anterior pore of the neural canal remains open, the archenteron communicates indirectly with the exterior (vide [fig. 2] E). It must not however be supposed (as has been done by some embryologists) that the pore at the front end of the neural canal represents the blastopore carried forwards. It is even probable that what Kowalevsky describes as the carrying of the blastopore to the dorsal side is really the commencement of the formation of the neural canal, the walls of which are continuous with the lips of the blastopore. This interpretation receives support from the fact that at a later stage, when the neural and alimentary canals become separated, the neural canal extends round the posterior end of the notochord to the ventral side. The embryonic communication between the neural and alimentary canals is common to most Chordata; and the tube connecting them will be called the neurenteric canal. It is always formed in fundamentally the same manner as in Amphioxus. With reference to the second point it is to be noted that Amphioxus is exceptional amongst the Chordata in the fact that, before the closure of the neural groove, the layer of cells which will form the neural tube becomes completely separated from the adjoining epiblast ([fig. 3] A), and forms a structure which may be spoken of as the medullary plate; and that in the closure of the neural canal the lateral epiblast forms a complete layer above this plate before the plate itself is folded over into a closed canal. This peculiarity will be easily understood from an examination of [fig. 3] A, B and C.
Fig. 3. Sections of an Amphioxus embryo at three stages. (After Kowalevsky.)
A. Section at gastrula stage.
B. Section of an embryo slightly younger than that represented in fig. 2 D.
C. Section through the anterior part of an embryo at the stage represented in fig. 2 E.
np. neural plate; nc. neural canal; mes. archenteron in A and B, and mesenteron in C; ch. notochord; so. mesoblastic somite.
The formation of the mesoblastic somites commences, at about the same time as that of the neural canal, as a pair of hollow outgrowths of the walls of the archenteron. These outgrowths, which are shewn in surface view in [fig. 2] B and D, so, and in section in [fig. 3] B and C, so, arise near the front end of the body and gradually extend backwards as wing-like diverticula of the archenteric cavity. As they grow backwards their dorsal part becomes divided by transverse constrictions into cubical bodies ([fig. 2] D and E), which, with the exception of the foremost, soon cease to open into what may now be called the mesenteron, and form the mesoblastic somites. Each mesoblastic somite, after its separation from the mesenteron, is constituted of two layers, an inner one—the splanchnic—and an outer—the somatic, and a cavity between the two which was originally continuous with the cavity of the mesenteron. Eventually the dorsal parts of the outgrowths become separated from the ventral, and form the muscle-plates, while their cavities atrophy. The cavity of the ventral part, which is not divided into separate sections by the above described constrictions, remains as the true body cavity. The ventral part of the inner layer of the mesoblastic outgrowths gives rise to the muscular and connective tissue layers of the alimentary tract, and the dorsal part to a section of the voluntary muscular system. The ventral part of the outer layer gives rise to the somatic mesoblast, and the dorsal to a section of the voluntary muscular system. The anterior mesoblastic somite long retains its communication with the mesenteron, and was described by Max Schultze, and also at first by Kowalevsky, as a glandular organ. While the mesoblastic somites are becoming formed the dorsal wall of the mesenteron develops a median longitudinal fold ([fig. 3] B, ch), which is gradually separated off from before backwards as a rod ([fig. 3] C, ch), underlying the central nervous system. This rod is the notochord. After the separation of those parts the remainder of the hypoblast forms the wall of the mesenteron.
With the formation of the central nervous system, the mesoblastic somites, the notochord, and the alimentary tract the main systems of organs are established, and it merely remains briefly to describe the general changes of form which accompany the growth of the larva into the adult. By the time the larva is but twenty-four hours old there are formed about seventeen mesoblastic somites. The body, during the period in which these are being formed, remains cylindrical, but shortly afterwards it becomes pointed at both ends, and the caudal fin appears. The fine cilia covering the larva also become replaced by long cilia, one to each cell. The mesenteron is still completely closed, but on the right side of the body, at the level of the front end of the mesenteron, the hypoblast and epiblast now grow together, and a perforation becomes formed through their point of contact, which becomes the mouth. The anus is probably formed about the same time if not somewhat earlier[4].
Fig. 4. Sections through two advanced embryos of Amphioxus to shew the formation of the peribranchial cavity. (After Kowalevsky.)
In A are seen two folds of the body wall with a prolongation of the body cavity. In B the two folds have coalesced ventrally, forming a cavity into which a branchial cleft is seen to open.
mes. mesenteron; br.c. branchial cavity; pp. body cavity.