This history is represented in the development of the dog-fish in an abbreviated form, by the elongation of the first segmental tube (segmental duct of the kidney) and its junction with each of the posterior segmental tubes. Professor Semper looks upon the primitive duct of the kidneys as a duct which arose independently, and was not derived from metamorphosis of the segmental organs. Against this view I would on the one hand urge the consideration, that it is far easier to conceive of the transformation by change of function (comp. Dohrn, _Functionswechsel_, Leipzig, 1875) of a segmental organ into a segmental duct, than to understand the physiological cause which should lead, in the presence of so many already formed ducts, to the appearance of a totally new one. By its very nature a duct is a structure which can hardly arise de novo. We must even suppose that the segmental organs of Annelids were themselves transformations of still simpler structures. On the other hand I would point to the development in this very duct amongst Amphibians and Osseous fishes of a glandular portion similar to that of a segmental tube, as an _à posteriori_ proof of its being a metamorphosed segmental tube. The development in insects of a longitudinal tracheal duct by the coalescence of a series of transverse tracheal tubes affords a parallel to the formation of a duct from the coalescence of a series of segmental tubes. Though it must be admitted that the loss of the external openings of the segmental organs requires further working out, yet the difficulties involved in their disappearance are not so great as to render it improbable that the vertebrate segmental organs are descended from typical annelidan ones. The primitive vertebrate condition, then, is probably that of an early stage of Selachian development while there is as yet a segmental duct,--the original foremost segmental tube opening in front into the body-cavity and behind into the cloaca; with which duct all the segmental tubes communicate. Vide Fig. 2. The next condition is to be looked upon as an indirect result of the segmental duct serving as well for the products of the generative organs as the secretions of the segmental tubes. As a consequence of this, the segmental duct became split into a ventral portion, which served alone for the ova, and a dorsal portion which received the secretion of the segmental tubes. The lower portion, which we have called the oviduct, in some cases may also have received the semen as well as the ova. This is very possibly the case with Ceratodus (vide Günther, _Trans. of Royal Society_, 1871), and the majority of Ganoids (Hyrtl, _Denkschriften Wien_, Vol. VIII.). In the majority of other cases the oviduct exists in the male in a completely rudimentary form; and the semen is carried away by the same duct as the urine.