THE URINOGENITAL ORGANS OF VERTEBRATES. 147 



finds that the segmental ducts develop first of all as simple 

 involutions from the body-cavity. The anterior end of each 

 then developes a glandular portion which comes to communicate 

 by a number of openings with the body-cavity. Subsequently 

 to the development of this glandular portion the remainder of 

 the kidneys appears in the posterior portion of the body-cavity ; 

 and before the close of embryonic life the anterior glandular 

 portion atrophies. 



The comparison of this system with that of a Selachian is 

 very simple. The first developed duct is the segmental duct of 

 a Selachian, and the glandular portion developed at its anterior 

 extremity, which is permanent in Myxine but embryonic in 

 Petromyzon, is, as W. Mu'ller has rightly recognized, equivalent 

 to the head-kidney of Amphibians, which remains undeveloped 

 in Selachians. It is, according to my previously stated view, 

 the glandular portion of the first segmental organ or the seg- 

 mental duct. The series of orifices by which this communicates 

 with the body-cavity are due to the division of the primary 

 opening of the segmental duct. This is shewn both by the facts 

 of their development in Petromyzon given by Miiller, as well as 

 by the occurrence of a similar division of the primary orifice in 

 Amphibians, which is mentioned later in this paper. In a note 

 in my original paper (loc. cif.) I stated that these openings 

 were equivalent to the segmental involutions of Selachians. 

 This is erroneous, and was due to my not having understood the 

 description given in a preliminary paper of Miiller (jfcnaisc/ie 

 ZeitscJirift, 1873). The large development of this glandular 

 mass in the Cyclostome and Osseous fishes and in embryo Am- 

 phibians, implies that it must at one time have been important. 

 Its earlier development than the remainder of the kidneys is 

 probably a result of the specialized function of the first seg- 

 mental organ. 



The remainder of the kidney in Cyclostomes is equivalent to 

 the kidney of Selachians. Its development from segmental in- 

 volutions has not been recognized. If these segmental involu- 

 tions are really absent it may perhaps imply that the simplicity 

 of the Cyclostome kidneys, like that of so many other of their 

 organs, is a result of degeneration rather than a primitive con- 

 dition. 



IO 2 



