The segmentation is followed by an invagination of nearly the same character as in Amphioxus. The blastosphere resulting from the segmentation first becomes flattened on one side, and the cells on the flatter side become more columnar ([fig. 8] I.). Very shortly a cup-shaped form is assumed, the concavity of which is lined by the more columnar cells. The mouth of the cup or blastopore next becomes narrowed; while at the same time the embryo becomes oval. The blastopore is situated not quite at a pole of the oval but in a position which subsequent development shews to be on the dorsal side close to the posterior end of the embryo. The long axis of the oval corresponds with the long axis of the embryo. At this stage the embryo consists of two layers; a columnar hypoblast lining the central cavity or archenteron, and a thinner epiblastic layer. The dorsal side of the embryo next becomes flattened ([fig. 8] II.), and the epiblast covering it is shortly afterwards marked by an axial groove continued forwards from the blastopore to near the front end of the body ([fig. 5], mg). This is the medullary groove, and it soon becomes converted into a closed canal—the medullary or neural canal—below the external skin ([fig. 6], n.c). The closure is effected by the folds on each side of the furrow meeting and coalescing dorsally. The original medullary folds fall into one another behind the blastopore so that the blastopore is situated on the floor of the groove, and, on the conversion of the groove into a canal, the blastopore connects the canal with the archenteric cavity, and forms a short neurenteric canal. The closure of the medullary canal commences at the blastopore and is thence continued forwards, the anterior end of the canal remaining open. The above processes are represented in longitudinal section in [fig. 8] III, n. When the neural canal is completed for its whole length, it still communicates by a terminal pore with the exterior. In the relation of the medullary canal to the blastopore, as well as in the closure of the medullary groove from behind forwards, the Solitary Ascidians agree closely with Amphioxus.
Fig. 6. 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.
The cells of the dorsal wall of the archenteron immediately adjoining the front and sides of the blastopore have in the meantime assumed a somewhat different character from the remaining cells of the archenteron, and give rise to a body which, when viewed from the dorsal surface, has somewhat the form of a horseshoe. This body was first observed by Metschnikoff. On the elongation of the embryo and the narrowing of the blastopore the cells forming this body arrange themselves as a broad linear cord, two cells wide, underlying about the posterior half of the neural canal ([fig. 7], ch). They form the rudiment of the notochord, which, as in Amphioxus, is derived from the dorsal wall of the archenteron. They are seen in longitudinal section in [fig. 8] II. and III. ch.
With the formation of the notochord the body of the embryo becomes divided into two distinct regions—a posterior region where the notochord is present, and an anterior region into which it is not prolonged. These two regions correspond with the tail and the trunk of the embryo at a slightly later stage. The section of the archenteric cavity in the trunk dilates and constitutes the permanent mesenteron ([figs. 7], al, and [8] III. and IV. dd). It soon becomes shut off from the slit-like posterior part of the archenteron. The nervous system in this part also dilates and forms what may be called the cephalic swelling ([fig. 8] IV.), and the pore at its anterior extremity gradually narrows and finally disappears. In the region of the tail we have seen that the dorsal wall of the archenteron becomes converted into the notochord, which immediately underlies the posterior part of the medullary canal, and soon becomes an elongated cord formed of a single or double row of flattened cells. The lateral walls of the archenteron ([fig. 7], me) in the tail become converted into elongated cells arranged longitudinally, which form powerful lateral muscles ([fig. 8] IV. m). After the formation of the notochord and of the lateral muscles there remains of the archenteron in the tail only the ventral wall, which according to Kowalevsky forms a simple cord of cells ([fig. 6], al). It is however not always present, or else has escaped the attention of other observers. It is stated by Kowalevsky to be eventually transformed into blood corpuscles. The neurenteric canal leads at first into the narrow space between the above structures, which is the remnant of the posterior part of the lumen of the archenteron. Soon both the neurenteric canal and the caudal remnant of the archenteron become obliterated.
Fig. 7. Optical section of an embryo of Phallusia mammillata. (After Kowalevsky.)
The embryo is of the same age as fig. 8 III, but is seen in longitudinal horizontal section.
al. alimentary tract in anterior part of body; ch. notochord; me. mesoblast.
During the above changes the tail becomes considerably elongated and, owing to the larva being still in the egg-shell, is bent over to the ventral side of the trunk.