THE VARIATIONS DURING LIFE 105 



" protoplasm " and a large store of unorganized nutritive material. 

 During development organization of this material takes place. The 

 amount of nuclear substances and living protoplasm is increased and 

 often enormously increased. At the same time some of the nutritive 

 material becomes catabolized, and in all eggs which are not nourished 

 from outside during development (all known eggs except those of 

 Mammals and Selachians) the total potential energy present is steadily 

 diminished. 



The old view of Pfliiger [i 868] that the respiratory exchange of the 

 embryo of a warm-blooded animal must be very small can now be dis- 

 missed without comment, the more so as it never had any experi- 

 mental foundation. The principles which must be guiding in the 

 study of embryonic metabolism were first enunciated by Bohr 

 [1900]. 



We have during the development of the embryo a rapid formation 

 of organized tissues. This formation may probably require the ex- 

 penditure of energy, but at the same time the tissues when formed 

 must have a certain metabolism, and the fundamental problem before us 

 is to distinguish as far as possible between these two components of 

 the embryonic metabolism, and to determine the " cost of production," if 

 any, of living " protoplasm " from unorganized chemical substances. 1 

 Bohr saw further that a certain amount of energy might possibly be trans- 

 ferred to the newly formed tissues whose energy content might be 

 higher because they were living, than that of the " dead " material from 

 which they were formed. If this latter possibility were realized the 

 direct and indirect methods of calorimetry must give different results, 

 the expenditure of energy as calculated from the respiratory exchange 

 being higher than that found by direct biocalorimetric determination 

 on embryos during development. In their classical paper, " Ueber 

 die Warmeproduktion und den Stoffwechsel des Embryos," Bohr and 

 Hasselbalch put this theoretical possibility to the test of experiment, 

 and found, as mentioned in detail on p. 1 2, that there was a complete 

 agreement between the metabolism as determined by direct and by 

 indirect calorimetry. Determinations of embryonic metabolism by 



1 The term " Entwickelungsarbeit " has been introduced and used by Tangl [1903] 

 in the important series of " Beitrage zur Energetik der Ontogenese". By " Entwickel- 

 ungsarbeit " Tangl denotes the total energy expended during embryonic life. The 

 determinations of the work of development have been made in most cases by compari- 

 sons of analyses (chemical and calorimetric) at the beginning and end of the incubation 

 period. 



