For this purpose I have not made use of my own observations alone, but have had recourse to all the Memoirs with which I am acquainted, and to which I have access. I have commenced my account with the Selachians, both because my own investigations have been directed almost entirely to them, and because their urinogenital organs are, to my mind, the most convenient for comparison both with the more complicated and with the simpler types. On many points the views put forward in this paper will be found to differ from those which I expressed in my paper (_loc. cit._) which give an account of my original[28] discovery of the segmental organs of Selachians, but the differences, with the exception of one important error as to the origin of the Wolffian duct, are rather fresh developments of my previous views from the consideration of fresh facts, than radical changes in them. Footnote 28: These organs were discovered independently by Professor Semper and myself. Professor Semper's preliminary account appeared prior to my own which was published (with illustrations) in the _Quarterly Journal of Mic. Science_. Owing to my being in South America, I did not know of Professor Semper's investigations till several months after the publication of my paper. * * * * * In Selachian embryos an intermediate cell-mass, or middle plate of mesoblast is formed, as in birds, from a partial fusion of the somatic and splanchnic layers of the mesoblast at the outer border of the protovertebræ. From this cell-mass the whole of the urinogenital system is developed. At about the time when three visceral clefts have appeared, there arises from the intermediate cell-mass, opposite the fifth protovertebra, a solid knob, from which a column of cells grows backwards to opposite the position of the future anus (Fig. 1. _pd._). [Illustration: FIG. 1. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL CLEFTS. The sections are to shew the development of the segmental duct (_pd_) or primitive duct of the kidneys. In _A_ (the anterior of the two sections) this appears as a solid knob projecting towards the epiblast. In _B_ is seen a section of the column which has grown backwards from the knob in _A_. _spn._ rudiment of a spinal nerve; _mc._ medullary canal; _ch._ notochord; _X._ string of cells below the notochord; _mp._ muscle-plate; _mp´._ specially developed portion of muscle-plate; _ao._ dorsal aorta; _pd._ segmental duct; _so._ somatopleura; _sp._ splanchnopleura; _pp._ pleuro-peritoneal or body-cavity; _ep._ epiblast; _al._ alimentary canal.]