The first change consists in the pushing in of a fold of skin, between the mouth and anus, towards the intestine, which at the same time rapidly elongates, and forms the axis of a conical projection, which thereupon becomes segmented and is thereby shewn to be the rudiment of the trunk (fig. 155 B). On the elongation of the trunk in this way the præ-oral lobe and its ciliated ring assume an appearance not very dissimilar to the same structures in Polygordius. At the ciliated apex of the præ-oral lobe a paired thickening of epiblast gives rise to the supra-oesophageal ganglia (_sg_). In the further metamorphosis, the præ-oral lobe and its ciliated ring gradually become reduced, and finally atrophy in the normal way, while the trunk elongates and acquires setæ. The dorsally situated processes with provisional setæ last for some time, but finally disappear. The young worm then develops a tube and shews itself as a normal tubicolous Chætopod. _Formation of Organs._ Except in the case of a few organs our knowledge of the formation of the organs in the Chætopoda is derived from investigations on the Oligochæta. The embryo of the Oligochæta has a more or less spherical form, but it soon elongates, and becoming segmented acquires a distinct vermiform character. The ventral surface is however for a considerable time markedly convex as compared to the dorsal. The ventrally placed mouth is surrounded by a well-marked lip, and in front of it is placed a small præ-oral lobe. [FIG. 156. SECTION THROUGH THE HEAD OF A YOUNG EMBRYO OF LUMBRICUS TRAPEZOIDES. (After Kleinenberg.) _c.g._ cephalic ganglion; _cc._ cephalic portion of the body cavity; _x._ oesophagus.] The epiblast. The epiblast cells at the commencement of the gastrula stage become much flattened, and on the completion of the invagination form an investment of flattened cells, only thickened in the neighbourhood of the mesoblastic bands (fig. 141 B and C). In the Polychæta at any rate the statements of several investigators would seem to indicate that the cuticle is derived from the chorion. It is difficult to accept this conclusion, but it deserves further investigation. Nervous system. The most important organ derived from the epiblast is the nervous system; the origin of which from this layer was first established by Kowalevsky (No. 342). [FIG. 157. SECTION THROUGH PART OF THE VENTRAL WALL OF THE TRUNK OF AN EMBRYO OF LUMBRICUS TRAPEZOIDES. (After Kleinenberg.) _m._ longitudinal muscles; _so._ somatic mesoblast; _sp._ splanchnic mesoblast; _hy._ hypoblast; _Vg._ ventral nerve cord; _vv._ ventral vessel.]