SECT. 7] OF EMBRYONIC DEVELOPMENT 967 



are required. This contrasts very markedly with the parallel calcula- 

 tion of Rubner for several mammals (horse, cow, sheep, pig, and 



dog) : Calories 



f/ or Ea. ... ... ... ... 2480 



M^ or specific energy content ... 1504 



3984 



One of the main differences between the two results is that the 



figures of Tangl were the result of a large amount of experimental 



work, whereas those of Rubner were approximations calculated on 



the basis of various more or less doubtful assumptions (see p. 494) . 



It followed that the efficiencies were divergent. The storage 



expressed in per cent, of the absorption of nourishment, Rubner's 



W 

 " energetischer Nutzungsquotient" or jj 7-7^ ~ 100, was, for Rubner's 



mammalian embryos, on an average 34-3, and for man was as low as 

 5-2, while, as will have already been noted from Table 121, for the 

 silkworm it was 63-2 and for the chick 67-0. Tangl concluded that in 

 the two latter cases quite other governing processes operate than in the 

 case of mammals, but it is probable that future work will not support 

 Rubner's "law". It is extremely difficult to see why intra-uterine 

 development should be so much less efficient than development within 

 an egg; the statement, indeed, has almost the status of a biological 

 paradox. At the same time, it would be interesting to know what 

 takes place as regards energy exchange in, for instance, an ovo-vivi- 

 parous selachian. 



It may be noted, however, that Rubner's law was found by Tangl 

 to hold quite well for the early post-embryonic life of the silkworm. 



15728 16020 



[t would, therefore, appear that Rubner's law holds in extra-uterine 

 or post-embryonic development only. If this is so, it is possible that 

 the efficiency of the mammalian embryo may be very like that of the 

 non-mammalian embryo, and in any case higher before than after 

 birth. 



A number of other workers have occupied themselves with the 

 energy relations of various embryos. Their experiments permit of the 



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