M. J. KARVONEN 



disorder in the solid phase, but the existence of some interactions in 

 the dissolved phase can also not be excluded. 



The long duration of the foetal-adult transition — about two months — 

 makes it rather probable, that the transition means a change in the 

 Hb synthesis, not a peripheral change in the circulating erythrocytes. 

 This view is also supported by the finding that in vitro the mixtures 

 apparently are quite stable. 



The erythrocytes of the foetus are histologically different from those 

 of an adult. The appearance of the adult type of Hb and the histo- 

 logical transition are not, however, strictly parallel. The transition of 

 the blood picture from foetal to adult is a gradual change, which is in 

 progress already before the appearance of the adult type of Hb. 



During the main period of the foetal life, the liver is the most 

 important site of blood formation. Bone marrow, however, attains 

 an increasing importance as an erythropoietic organ towards the end 

 of the foetal life. But again, there is no parallelism between the 

 development of bone marrow and the appearance of the adult type 

 Hb, because the bone marrow in the sheep foetus contributes in- 

 creasingly to the erythropoiesis already from the age of approximately 

 60 days onwards. The causal mechanism of the transition remains an 

 unsolved question. 



SUMMARY 



/ In adult sheep the haemoglobin (Hb) is salted out approximately 

 according to Cohn's equation, but if a constant solvent solubility 

 study is made by varying the concentration of the protein, the Hb 

 behaves in a way, which suggests the presence of two probably nearly 

 related components. 



2 The same is true of the Hb of foetal sheep before the foetal age 

 of about 115 to 120 days. The solubility of foetal Hb is, however, 

 widely different from that of the adult Hb ; at pH 7-2 the adult Hb is 

 about 20 times more soluble than the foetal one. 



3 During the last month of the foetal life and the first month of 

 the post-natal fife the solubility of the Hb is analogous to the solubility 

 of artificial mixtures made of adult and foetal Hb's. The solubility is 

 generally higher than additive. The solubility of artificial mixtures 

 can be used as a purely empirical basis for the approximate estimation 

 of the percentage composition of the Hb during the transition period. 



4 In exceptional cases the transition may result in the production 

 of a third type of Hb. 



5 The causative mechanism of the transition remains unknown. 



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