FCETAL NUTRITION: THE PLACENTA 511 



In addition, we possess positive evidence of metabolic activity in 

 the mammalian ovum. The results of Bohr's investigations on the 

 respiratory exchange of the foetus (see p. 464) mean nothing if they 

 do not afford proof of this. As a large amount of energy is generated, 

 while, at the same time, practically none is dissipated as heat 

 evaporated or radiated from the surface or lungs, the unavoidable 

 conclusion is that the foetus itself carries out the work of organisa- 

 tion, and utilises the energy for its fulfilment. 



When we come to consider individual substances, we obtain 

 further evidence of activity, at least in the extra-embryonic ectoderm 

 or trophoblast. In no order of Mammals has the transmission of 

 haemoglobin as such from mother to foetus been demonstrated. Even 

 if it is absorbed as such by the trophoblast, it undergoes changes of 

 such a nature that the iron-containing part of the molecule is less 

 firmly bound. In all animals in which special investigations have 

 been made, such loose organic compounds of iron have been observed. 

 In this connection, reference may once more be made to Hofbauer's 

 statement that the histological appearances argue not only for a 

 decomposition of maternal haemoglobin in the syncytium, but also for 

 a synthesis of its derivatives into organic compounds in which the iron 

 is more firmly bound (see p. 505). 



The trophoblast probably acts also on simpler proteins. If ox- 

 serum is injected into a pregnant rabbit, its proteins cannot be 

 detected by the biological reaction in the semm of the foetus. It 

 may be that the trophoblast rejects them altogether, but this is 

 unlikely, since molecules of egg-albumen are absorbed and transferred 

 to the foetus (see p. 464). In all probability the proteins of ox-serum 

 are katabolised in the villi, and, as a result, the constitution of the 

 precipitable substance is interfered with, and the precipitin reaction 

 is negative. The existence of an intra-cellular proteolytic enzyme l 

 and decomposition products of the proteins in the placenta also point 

 to the occurrence of a trophoblastic metabolism of proteins. 



The carbohydrates undergo changes which appear to be the result 

 of 'trophoblastic activity. In the rabbit, the glycogen which is 

 "swallowed" along with the decidual cells by the plasmodium 

 (Chipman) is not found as glycogen. A hydrolytic transformation 

 to sugar probably takes place (see p. 462). In addition the foetal 

 serum contains Irevulose, which must be formed in some part of the 

 fertilised ovum, since it is absent in the mother. Fats may also be 

 transformed by the trophoblast (see Chapter XI., p. 542). 



1 It is desirable that the presence of the enzyme in the trophoblast should 

 be established, and this can only be done in such animals as the sheep and 

 rabbit, in which the foetal placenta can be detached from the maternal, and 

 investigated separately. As was previously mentioned, the placenta contains 

 uo extra-cellular proteolytic enzyme. 



