CHAPTER XII. 

 THE ORGANS OF EXCRETION. 



THE earliest stages in the development of the excretory 

 system have already been described in a previous chapter 1 of this 

 memoir, and up to the present time no investigator, with the 

 exception of Dr Alex. Schultz 2 , has gone over the same ground. 

 Dr Schultz' descriptions are somewhat brief, but differ from my 

 own mainly in stating that the segmental duct arises from an 

 involution instead of as a solid knob. This discrepancy is, 

 I believe, due to Dr Schultz drawing his conclusions as to the 

 development of the segmental duct from its appearance at a 

 comparatively late stage. He appears to have been unac- 

 quainted with my earlier descriptions. 



The adult anatomy and later. stages in the development of 

 the excretory organs form the subject of the present chapter, 

 and stand in marked contrast to the earlier stages in that they 

 have been dealt with in a magnificent monograph 3 by Professor 

 Semper, whose investigations have converted this previously 

 almost unknown field of vertebrate embryology into one of the 

 most fully explored parts of the whole subject. Reference is 

 frequently made to this monograph in the succeeding pages, but 

 my references, numerous as they are, give no adequate idea of 

 the completeness and thoroughness of Professor Semper's in- 

 vestigations. In Professor Semper's monograph are embodied 

 the results of a considerable number of preliminary papers pub- 

 lished by him in his Arbeiten and in the Centralblatt. The 

 excretory organs of Elasmobranchs have also formed the sub- 



1 Chapter vi. p. 345, et seq. 



2 Archiv f. Micr. Anat. Bd. XI. 



3 " Urogenital System d. Plagiostomen," Semper, Arbeiten, Vol. II. 



