102 E. R. HOSKINS AND M. M. HOSKINS 



Nearly every reasonable function has been ascribed to the 

 digitiform gland, but usually from morphological or hypothetical 

 rather than physiological evidence (Hoskins, '17). The gland 

 is a compound tubular structure with a large central lumen or 

 duct which empties into the intestine about half way between 

 the spiral valve and the cloaca. It is suspended in the mesen- 

 tery above the intestine. Cytologically, the gland is not unlike 

 the kidney tubules, and this, together with the fact that its 

 secretion is discharged into the intestine posterior to the region 

 where digestion occurs, leads to the expectation that it has an 

 excretory function. The fact that the gland appears to be the 

 same in both sexes argues against the probabihty that it is 

 accessory to the sexual apparatus. 



Another part of the work is a study of the function of the 

 kidney (mesonephros) compared with that of mammals (meta- 

 nephros). It is common knowledge that the former is a less 

 efficient excretory organ than the latter, but the subject has 

 never been completely studied experimentally. A study is 

 made also of the excretory function of the liver, which is of 

 considerable importance in selachians, on account of the ineffi- 

 ciency of the kidney. In selachians the liver undergoes fatty 

 metamorphosis to such extent that nearly every cell appears 

 filled with fat, yet the organ is able to excrete solutions and 

 particles freely and in large amounts. 



Denis ('13) has shown that dogfish are able to withstand large 

 doses of excretory toxins, but this author did not study the 

 problem histologically. The question of vital staining is con- 

 sidered here only incidentally and no attempt is made to describe 

 completely the reaction of the dogfish to vital stains. These 

 substances were used primarily as an aid to the study of the 

 excretory function. 



Most of the experiments were performed at the Marine Bio- 

 logical Laboratory at Woods' Hole, Massachusetts, in quarters 

 provided us through the kindness of the Department of Zoology 

 of Yale University and of Dr. Frank Lilhe. The accompanying 

 photomicrographs were made at the Osborn Zoological Labora- 

 tory of Yale University. Two experiments were done at the 



