SECT. 9] 



PROTEIN METABOLISM 



1117 



Evidently the dogfish embryo excretes its urea into its yolk, and so 

 retains it for its osmotic purposes. This peculiarity, then, made it 

 possible to regard the egg as a closed system, and to estimate the 

 amounts of urea produced by the embryo at different ages, the results 

 of which are shown in Fig. 338. By the end of development some 

 15 mgm, of urea are present, but, as we do not yet know the exact 

 amount of protein nitrogen present at the beginning, we cannot say 

 what percentage this is. 



Unfortunately, no good series of weighings exists for Scyllium em- 

 bryos, but we know from the work of 

 Fulton and Kearney that the length 

 of fish embryos increases at first far 

 more rapidly than the weight. Now 

 this evidently implies that when the 

 points are plotted against weight, the 

 slight concavity towards the abscissa, 

 shown in Fig. 338, will be greatly |' 

 accentuated, because equal incre- 

 ments of length mean much greater 

 increments of weight in the later stages 

 than in the earlier. Plotted against 

 weight, then, the urea-content curve 

 would rise sharply to a certain point 

 and then very slowly. Accordingly 

 when the urea produced by the em- 

 bryo is referred to unit weight of embryo, a peaked curve would 

 result. 



This expectation we found to be fulfilled as far as possible when we 

 used as weight data the figures of Kearney for Mustelus canis. We laid 

 no emphasis on the result though it will be admitted that the shape 

 of the ascending weight/age curve for Mustelus probably does not 

 differ much from that of the related Scyllium. As the last column of 

 the following figures demonstrates, a descending curve is obtained 

 over the range covered by Kearney. 



The excretion of urea into its yolk by the elasmobranch embryo 

 may perhaps throw light on some of the morphological results of 

 Borcea, who studied in detail the development of the urinogenital 

 system in these fishes. 



Summing up what we know of the protein catabolism of the fish 



