2 1(5 METAMORPHOSIS OF TISSUE. 



As this method did not afford any prospect of deciding the 

 question of the formation of fat from protein- substances in the 

 animal body, Burdach has attempted another method, which, as 

 far as we are able to judge at present, appears likely to furnish 

 definite results. Comparative analyses of undeveloped and already 

 developed ova have been instituted, with a view of ascertaining 

 the increase or decrease of certain salts or of sugar during the 

 development of the embryo ; and the first question which 

 presents itself for consideration is whether the embryo does or 

 does not contain more fat and less protein-bodies than the egg 

 from which it is developed. If the amount of fat in the egg 

 increased in a definite ratio to the diminution of the protein-sub- 

 stance, or of the nitrogenous matters generally, the conversion of 

 protein- matter into fat during the metamorphosis of matter would 

 be demonstrable at all events for this case. Burdach certainly 

 found in some experiments which he made on the ova of the 

 Limnseus stagnalis, that there was no inconsiderable increase of fat 

 during the development of the embryo ; but as these few obser- 

 vations did not coincide perfectly as to their results, the question 

 must still be regarded as undecided. 



The third group of the most important substances of the animal 

 body (comprising the carbo-hydrates) stands in such an intimate 

 relation to fat, that in noticing these bodies we shall at the same 

 time have occasion to make many additional remarks regarding the 

 value of fat in the animal economy. We are only acquainted with 

 four substances of this group as constituents of the animal body, 

 namely dextrin, milk-sugar, inosite, and grape-sugar (glucose ). 

 We have already seen that the carbo-hydrates (with the exception 

 of the cellulose deposited in the outer integuments of the Tunicata, 

 (see vol.i. p. 299) never constitute the basis of tissues. Sugar which 

 was formerly considered to be almost limited to the primoe vise, has 

 recently been discovered in nearly all the animal fluids which con- 

 tribute towards nutrition, such as the blood, the transudations, 

 lymph, chyle, the white of egg, &c. The sugar which we find in 

 the intestinal canal of the herbivora and omnivora owes its origin 

 to the metamorphosis of the starch and other carbo-hydrates 

 through the influence of the saliva and pancreatic juice; but sugar 

 is also met with in no very inconsiderable amount in the blood of 

 the carnivora (see vol. ii, p. 211), and must, therefore, be 

 dependent upon some other source besides the carbo-hydrates 

 which are introduced into the body from without. I have been 

 enabled by a number of comparative analyses of the blood of the 



