136 THE URINOGENITAL ORGANS OF VERTEBRATES. 



The following paper is an attempt to give a consecutive 

 history of the origin of this system of organs in vertebrates and 

 of the changes which it has undergone in the different orders. 



For this purpose I have not made use of my own observa- 

 tions alone, but have had recourse to all the Memoirs with which 

 I am acquainted, and to which I have access. I have com- 

 menced 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. V.) which give an account of my original 1 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. 



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 protovertebrse. 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. \,pd.}. 



This knob projects outwards toward the epiblast, and the 

 column lies at first between the mesoblast and epiblast. The 

 knob and column do not long remain solid. The knob be- 

 coming hollow acquires a wide opening into the pleuroperitoneal 

 or body cavity, and the column a lumen ; so that by the time 

 that five visceral clefts have appeared, the two together form a 



1 These organs were discovered independently by Professor Semper and myself. 

 Professor Semper's preliminary account appeared prior to my own which was pub- 

 lished (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 yaper. 



