570 THE RESPIRATORY EXCHANGE 



ture an increase, in its respiratory exchange. For a short time 

 before hatching there is an intermediate condition during which 

 a change of temperature has no marked effect and at last when 

 the chick is hatched it responds as a warm-blooded animal to 

 changes of external temperature. 



The subject of foetal respiration affords one of the most 

 interesting examples of the way in which correct explanations 

 well founded upon observation may after the lapse of years be 

 either forgotten or replaced by erroneous theories until they are 

 rediscovered after a further lapse of years. Mayow maintained, 

 in his account of the foetal respiration published in 1674, that the 

 placenta is to be regarded as a lung, by which the umbilical vessels 

 take up nitro-aerial gas, or oxygen, and convey it to the foetus ; 

 he compared the condition of the foetus to that of apnoea, and 

 also described the process of absorption of oxygen by the blood 

 in the gills of fishes and in the embryo chick. This view of the 

 foetal respiration was adopted and extended by Hulse and by 

 Ray, who makes the following clear statement in the twelfth edition 

 of his work, " The Wisdom of God in the Creation," published in 

 1759 : " The maternal blood which flows to the cotyledons and 

 encircles the papillae communicates by them to the blood of the 

 foetus the air wherewith itself is impregnate ; as the water flowing 

 about the carneous radii of the fish's gills doth the air that is 

 lodged therein to them." 



Scheel, Jeffray, Bostock, and others observed that the blood 

 in the umbilical vein was brighter than that in the umbilical 

 artery, and thus supplied further evidence of the respiratory 

 function of the placenta. Confusing evidence, however, based 

 upon faulty observations, had arisen in the meantime. Johannes 

 Miiller, who is often called, and rightly so, " The Father 

 of Modern Physiology," held that plasma or lymph passed from 

 the mother to the foetus, and so supplied the place of respiration ; 

 there was no difference, he maintained, in the colour of the blood 

 in the umbilical artery and umbilical vein, and he even considered 

 it necessary to test by experiment the view that the foetus breathed 

 by absorbing oxygen from the amniotic fluid by means of its skin 

 or lungs. Such was the condition of knowledge about a century 

 and a half after the death of Mayow ; his brilliant work had been 

 forgotten or neglected, and the correctness of his view that the 



