They are simple independent outgrowths arising from the outer and uppermost angle of the body-cavity, and are at first almost without a trace of a lumen; though their cells are arranged as two layers. They grow in such a way as to encircle the oviduct on its inner and upper side (Pl. 11, fig. 8 and Pl. 12, fig. 14_b_, _st_). When the hindermost ones are formed, a slight trace of a lumen is perhaps visible in the front ones. At a stage slightly subsequent to this, in Scyllium canicula, I noticed 29 of them; the first of them arising in the segment immediately behind the front end of the oviduct (Pl. 12, fig. 17, _st_), and two of them being formed in segments just posterior to the hinder extremity of the oviduct. Pl. 12, figs. 16 and 18 represent two longitudinal sections shewing the segmental nature of the involutions and their relation to the segmental duct. Many of the points which have been mentioned can be seen by referring to Pl. 11 and 12. Anteriorly the segmental duct opens into the pleuro-peritoneal cavity. In the sections behind this there may be seen the segmental duct with a distinct lumen, and also a pair of segmental involutions (Pl. 12, fig. 14_a_). In the still posterior sections the segmental duct would be quite without a lumen, and would closely adjoin the epiblast. It seems not out of place to point out that the modes of the development of the segmental duct and of the segmental involutions are strikingly similar. Both arise as solid involutions, from homologous parts of the mesoblast. The segmental duct arises in the vertebral segment immediately in front of that in which the first segmental involution appears; _so that the segmental duct appears to be equivalent to a single segmental involution_. The next stage corresponds with the first appearance of the external gills. The segmental duct now communicates by a wide opening with the body-cavity (Pl. 11, fig. 9, _sd_). It possesses a lumen along its whole length up to the extreme hind end (Pl. 11, fig. 9_a_). It is, however, at this hinder extremity that the most important change has taken place. This end has grown downwards towards that part of the alimentary canal which still lies behind the anus. This downgrowth is beginning to shew distinct traces of a lumen, and will appear in the next stage as one of the horns by which the segmental ducts communicate with the cloaca (Pl. 11, fig. 9_b_). All the anterior segmental involutions have now acquired a lumen. But this is still absent in the posterior ones (Pl. 11, fig. 9_a_). Owing to the disappearance of the body-cavity in the region behind the anus, the primitive involutions there remain as simple masses of cells still disconnected with the segmental duct (Pl. 11, figs. 9_b_, 9_c_ and 9_d_). _Primitive Ova._ The true generative products make their first appearance as the _primitive ova_ between stages I and K.