iii6 PROTEIN METABOLISM [pt. iii 



were excreted through the kidneys of an adult dogfish per kilo per 

 day. The gills have been found by Duval & Portier to be absolutely 

 impermeable to urea. But van Slyke & White found that large 

 amounts of urea were contained in the bile (up to 72-3 per cent, of 

 the total biliary nitrogen), so that the intestinal tract is probably 

 concerned together with the liver cells in regulating the urea-content. 

 The kidney certainly does not seem to do much regulation, as Denis 

 found the blood urea to be uninfluenced by experimental nephritis 

 induced by uranium nitrate or potassium chromate. Another mode 

 of elimination of urea from the elasmobranch body is through the 

 peritoneal pores, which Smith believes to have an excretory function, 

 for the peritoneal fluid of a dogfish contains 680 mgm. per cent. 



More recently, Needham & Needham made an investigation of 

 urea production in the embryos of Scyllium canicula and Pristiurus 

 melanostoma. We first of all carried out a series of experiments to 

 ascertain whether the dogfish tgg was a closed system, and, by 

 allowing eggs to remain in comparatively restricted amounts of sea 

 water, we found that not more than traces of urea are lost to the 

 exterior, no matter whether the embryo be less than i cm. long, 

 or more than 7, i.e. nearly ready to hatch. Rather complicated 

 controls were here necessary as the diatoms in sea water destroy urea, 

 a fact insufficiently taken into account by earlier workers. By con- 

 structing osmometers of the thick horny egg-shells, we further found 

 that, although they themselves were permeable to urea in the out- 

 ward direction yet the sHmy coat on the inside effectually made 

 them impermeable. But the fact that the egg was a closed system 

 as regards nitrogenous end products led to a paradox, for, as is well 

 known, the egg-cases of the dogfish possess from the beginning four 

 slits at one end, which, at first plugged with albumen, open to 

 the sea water about two-thirds of the way through development. 

 It is even probable that a current of sea water may pass through 

 them, introduced by the waving of the embryo's external gills. Why, 

 then, does not the urea escape? We found the answer when we 

 estimated the urea in yolk, egg-white, and embryo separately: 



Urea 

 (mgm.) 



0'5 cm. embryo and yolk 8-32 



Corresponding white ... ... o- 1 65 



3-0 cm. embryo and yolk ... ... ii"59 



Corresponding white ... ... 0-03 



