Amphioxus. The mesoblast originates in Amphioxus, as in several primitive invertebrate types, from a pair of lateral diverticula, constricted off from the archenteron ([fig. 180]). Their formation commences at the front end of the body and is thence carried backwards, and each diverticulum contains a prolongation of the cavity of the archenteron. After their separation from the archenteron the dorsal parts of these diverticula become divided by transverse septa into successive somites, the cavities of which eventually disappear; while the walls become mainly converted into the muscle-plates, but also into the tissue around the notochord which corresponds with the vertebral tissue of the higher Chordata.
The ventral part of each diverticulum, which is prolonged so as to meet its fellow in the middle ventral line, does not become divided into somites, but contains a continuous cavity, which becomes the body cavity of the adult. The inner layer of this part forms the splanchnic mesoblast, and the outer layer the somatic mesoblast.
The notochord would almost appear to arise as a third median and dorsal diverticulum of the archenteron ([fig. 180] ch). At any rate it arises as a central fold of the wall of this cavity, which is gradually constricted off from before backwards.
Fig. 181. Transverse optical section of the tail of an embryo of Phallusia mammillata. (After Kowalevsky.)
The section is from an embryo of the same age as fig. 8 IV.
ch. notochord; n.c. neural canal; me. mesoblast; al´. hypoblast of tail.
Urochorda. In simple Ascidians the above processes undergo a slight modification, which is mainly due (1) to a general simplification of the organization, and (2) to the non-continuation of the notochord into the trunk.
The whole dorsal wall of the posterior part of the archenteron is converted into the notochord ([fig. 181] ch), and the lateral walls into the mesoblast (me); so that the original lumen of the posterior part of the archenteron ceases to be bounded by hypoblast cells, and disappears as such. Part of the ventral wall remains as a solid cord of cells (al´) The anterior part of the archenteron in front of the notochord passes wholly into the permanent alimentary tract.
The derivation of the mesoblast from the lateral walls of the posterior part of the archenteron is clearly comparable with the analogous process in Amphioxus.