44 ADAM SEDGWICK. 



and in the present state of our knowledge no satisfactory ex- 

 planation, founded on facts of development, can be given of it. 

 I will suggest a possible, but entirely rough and hypothetical, 

 solution on the lines so far followed. 



Before the Elasmobranchii produced eggs with the large food 

 yolk they at present possess, they may have undergone a large 

 part of their development in the surrounding medium as free 

 larvae. These larvse must have left the egg at a time when 

 the cavities of the muscle plates were still open to the body 

 cavity, and when the segmental duct had only just commenced 

 to be formed in front, and before the development of the vascular 

 system, and therefore before the glomerulus, the functions of 

 which were probably carried on by the walls of the body cavity. 

 The segmental duct was quickly developed from a groove into a 

 duct, the larvae thus precociously developing a recently acquired 

 adult structure. With this constitution the larva of the ances- 

 tral Elasmobranch quickly developed the rest of its excretory 

 system. In consequence of the larva having been hatched at a 

 very primitive stage, before the muscle plates were separated 

 from the body cavity, certain primitive characters in the develop- 

 ment of the segmental tubes were retained. These characters 

 have been more or less transmitted to the present day, this 

 having been rendered possible by the acquisition of food yolk 

 and abolition of the larval state. 



However this may be, and it is useless now to make hypo- 

 theses of this kind, we can only wait till a more close study of 

 Elasmobranch development has been made to see if any traces 

 can be found of the disturbing cause which has produced the 

 modification in the development of the excretory system assumed 

 on the above hypothesis, and very possibly in the search along 

 the lines which this hypothesis indicates quite a difi'erent view as 

 to the phylogeny of the vertebrate excretory system may pre- 

 sent itself. 



Before concluding I will briefly state what I think to have 

 been the structure of the primitive excretory system in the 

 ancestral Yertebrate. 



There was a duct occupying the position of the segmental 

 duct, i. e. at the dorsal outer angle of the body cavity, at the 

 point where the latter becomes separated from the cavities of 

 the muscle plates. This duct opened in each segment into the 

 dorsal part of the body cavity. On the inner wall of the latter 

 projected on each side a vascular ridge formed by the aorta. 

 Behind, the segmental duct opened into the cloaca. 



As differentiation proceeded the vascular aortic ridge became 

 more especially developed opposite each opening of the segmental 

 duct, and parts of each of these enlargements became succes- 



