138 PROFESSOR MARSHALL AND EDWARD J. BLES. 



most anterior myotome in the frog extends forwards to the level of 

 the hinder border of the auditory vesicle in a tadpole of about i\ mm. 

 length. The first nephrostome lies immediately below the ventral 

 border of the second myotome, close to its posterior edge; the 

 second nephrostome is rather more dorsally placed, and lies below 

 the ventral border of the third myotome, rather nearer its posterior 

 than its anterior edge ; and the third nephrostome is similarly 

 related to the fourth myotome. 



The nephrostomes are at first mere holes leading from the body 

 cavity into the dorsal part of the tubular head kidney. With 

 growth of the head kidney, these holes become drawn out into 

 tubes, which may conveniently be spoken of as the nephrostomial 

 tubes or tubules (Fig. 6, NT). These nephrostomial tubules differ 

 in many respects from the ordinary tubules of which the rest of the 

 head kidney consists ; they are of much smaller calibre, and their 

 walls consist of a very thin outer connective tissue coat, within 

 which is a single layer of cubical epithelial cells, which have a 

 curiously clear, transparent appearance, have strongly marked 

 and pigmented walls, and bear long fiagella, which project into 

 the tubule, with their free ends directed away from the body 

 cavity. 



The duct of the head kidney acquires its opening into the cloaca 

 in tadpoles between 4| mm. and 5 mm. in length, i.e., just previous 

 to the time of hatching. The terminal portion of the duct appears 

 to us to be developed rather as a lateral outgrowth from the cloaca 

 than by further growth on the part of the duct itself. 



Concerning the nomenclature of this duct, it is exceedingly difficult 

 to be consistent. At the time the tadpole hatches, the head kidney 

 is a well-developed convoluted tubular gland, and is the only renal 

 organ present ; the duct is clearly connected with it, and with it 

 alone ; and the closeness of the connection is emphasized by the con- 

 tinuity of the two structures from the time of their first appearance. 

 It seems, therefore, reasonable to speak of the duct as the duct of 

 the head kidney, or pronephric duct. Difficulties, however, arise 

 in the later stages, when the hinder part of the duct receives the 

 tubules of the "Wolffian body ; while in the adult frog the head kidney 

 disappears completely, and the posterior part of the duct persists in 

 both sexes as the ureter. Duct of the head kidney, or pronephric 



