SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 617 



oxygen tension than ordinary air. Von Baer, who was at that time 

 writing the introductory parts of his immortal book on the embryology 

 of the chick, was at once interested, for, just as Haldane a century 

 later was to welcome the secretory theory of the pulmonary epithelium 

 as a basis for views of a vitalistic character, so von Baer saw in the 

 analyses of Bischof and Dulk a like support for his general opinions. 

 On p. 37 he considered the gases of the egg, and, referring to a 

 figure of 27 per cent, of oxygen (atmospheric air 20-5-2 i-o), said, 

 "This value was Dr Bulk's. Previously Dr Bischof paid attention 

 to the air in the egg and found amounts of from 22 to 25 per cent. 

 Although his paper was rather short, I hope Dr Dulk will repeat his 

 observations. The result of this research is for embryology and the 

 whole of physiology so important that I feel it my duty to make very 

 well known these valuable communications". Nothing, however, has 

 been done on the problem since that time. But in 1847 Baudrimont & 

 Martin de St Ange examined the air in the air-space, though it must 

 be admitted that their analytical figures showed as often as not 

 less oxygen in the air-space, not more, than in atmospheric air. 



The most surprising feature of the early work, however, was the 

 fact that a number of workers seriously suggested that the hen's 

 egg was quite independent of air during its development. The 

 main upholders of this doctrine were Erman; Viborg and Towne. 

 Reaumur in the eighteenth century had already found (see p. 198) 

 that development would not go on if the egg was covered with some 

 substance impermeable to air, but Erman and Viborg, using gypsum, 

 got different results and affirmed that air was quite unnecessary, 

 for they could hatch out chicks from eggs buried in this way. Towne 

 came to the same conclusion from experiments in which he gummed 

 pieces of paper all over the outside of the shell. Von Baer, as one of 

 his footnotes shows, was prepared to accept a good deal of this work, 

 for it fitted in with his own idea that there was some essential difference 

 between pre-natal and post-natal development. He anticipated, it 

 would almost seem, the discovery of two quite different sorts of 

 metabolism. In 1834, however, Theodor Schwann, destined to be 

 known later on as the first cytologist, proved clearly that air was 

 essential for development, by maintaining eggs in an atmosphere of 

 hydrogen. His inaugural thesis at Berlin, De necessitate aeris atmo- 

 sphaerici ad evolutionem pulli in ovo incubato, finally settled the question, 

 though for some time other workers, such as Marshall, thought it 



