SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 619 



to hatch out a large number of eggs which had been varnished over 

 the air-space in this way. He could not confirm Dareste's statement 

 that the air-space could move round to one side after varnishing. 



Busing then proceeded to ascertain exactly how much of the 

 shell surface was requisite for full oxygenation of the embryo. By 

 varnishing little squares in chessboard formation of exactly equal 

 area all over the egg, he found that certainly 50 per cent, of the 

 shell could be occluded without any ill results being produced. In 

 one case, perfectly normal development followed the occlusion of 65 

 to 70 per cent, of the total surface in this way, but the number of 

 abnormal embryos rose rather rapidly at this point. Gerlach & Koch 

 went even further, and varnished eggs over the entire surface save for 

 a small circle from 4 to 6 mm. in diameter as near as they could judge 

 immediately over the germinal disc. The embryos produced in eggs 

 so treated were often abnormal, but seemed occasionally to be well 

 developed ; in all cases, however, they were much smaller and lighter 

 than the normal. The nearer the "Luftfleck" was to the embryo, 

 the more normal the development. Preyer drew from these experi- 

 ments the conclusion that a proper supply of oxygen must be more 

 essential for growth than for differentiation, but he did not follow 

 out that interesting line of thought, which has affinities with Demoor's 

 finding that irrespirable gases stop cytoplasmic streaming in Trades- 

 cantia, but not nuclear division (see also p. 542). Gerlach, who 

 studied in morphological detail the embryos resulting from this 

 method, found that the abnormalities must in many cases have arisen 

 during the first 15 hours of development, a fact which demonstrated 

 that, even in those early stages, oxygen was necessary. Varnishing 

 experiments were afterwards continued by Fere and by Mitrophanov 

 and low-pressure work by Giacomini. For the complicated early 

 history of the work on the respiration of eggs, with all its details, 

 Dareste's 1861 paper should be consulted. 



Baudrimont & Martin de St Ange made one experiment of much 

 interest, in view of the later work of Riddle (see Section 18-9), in which 

 they maintained eggs at 37° in an atmosphere of 85 per cent, oxygen. 

 They found on opening the eggs after some days that the embryo 

 had a red colour, the allantois was a millimetre thick and very 

 resistant, and the amniotic liquid was red, owing to the presence 

 of numbers of erythrocytes in it. Pott & Preyer later repeated this 

 experiment, and found that the description of the earlier workers 



