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. [FIG. 8. VARIOUS STAGES IN THE DEVELOPMENT OF PHALLUSIA MAMMILLATA. (From Huxley; after Kowalevsky.) The embryos are represented in longitudinal vertical section. I. Commencing gastrula stage. _fh._ segmentation cavity. II. Late gastrula stage with flattened dorsal surface. _eo._ blastopore; _ch._ notochord; _dd._ hypoblast. III. A more advanced embryo with a partially-formed neural tube. _ch._ and _dd._ as before; _n._ neural tube; _c._ epiblast. IV. Older embryo in which the formation of the neural tube is completed. _dd._ hypoblast enclosing persistent section of alimentary tract; _dd´._ hypoblast in the tail; _m._ muscles. V. Larva just hatched. The end of the tail is not represented. _a._ eye; _gb._ dilated extremity of neural tube with otolith projecting into it; _Rg._ anterior swelling of the spinal division of the neural tube; _f_. anterior pore of neural tube; _Rm._ posterior part of neural tube; _o._ mouth; _Chs._ notochord; _kl._ atrial invagination; _dd._ branchial region of alimentary tract; d. commencement of oesophagus and stomach; _dd´._ hypoblast in the tail; _m._ muscles; _hp._ papilla for attachment. VI. Body and anterior part of the tail of a two days' larva. _klm._ atrial aperture; en. endostyle; _ks._ branchial sack; _1ks. 2ks._ branchial slits; _bb._ branchial vessel between them; _ch._ axial portion of notochord; _chs._ peripheral layer of cells. Other reference letters as before.] The larva at this stage is represented in a side view in fig. 8 IV. The epidermis is formed throughout of a single layer of cells. In the trunk the mesenteron is shewn at _dd_ and the dilated part of the nervous system, no longer communicating with the exterior, at _n_. In the tail the notochord is shewn at _ch_, the muscles at _m_, and the solid remnant of the ventral wall of the archenteron at _dd´_. The delicate continuation of the neural canal in the tail is seen above the notochord at _n_. An optical section of the tail is shewn in fig. 6. It is worthy of notice that the notochord and muscles are formed in the same manner as in Amphioxus, except that the process is somewhat simplified. The mode of disappearance of the archenteric cavity in the tail, by the employment of the whole of its walls in the formation of various organs, is so peculiar, that I feel some hesitation in accepting Kowalevsky's statements on this head[6]. [6] It is more probable that this part of the alimentary tract is equivalent to the postanal gut of many Vertebrata, which is at first a complete tube, but disappears later by the simple absorption of the walls.