130 Scientific Proceedings j Roi/id Dublin Society. 



fall would, according to the above view, take place as the yolk travels down 

 tlie oviduct. 



(c) It is possible that both the yolk as formed in tlie ovary and the 

 albuminous secretions of the oviduct are isotonic, both being deficient in 

 chlorides. In any case it is found that, when the two layers of membrane 

 and the shell have been formed around the egg, the osmotic pressures of 

 the yolk aTid white are the same. Tlie two membranes together are semi- 

 permeable; but it was not ascertained which of them prevents the passage 

 of salts, or wlietlier both act. The deposition of the shell, which is nearly 

 pure calcium carbonate, must be a great drain upon the supply of calcium 

 in the blood feeding the shell-glands. At any rate the amount of calcium 

 found in the interior of an egg is small, only 0'04 per cent., or 0*023 grm., 

 in an egg weighing 52"84 grm. without the shell, which weighed 6'18 grm. 



It has recently been proved (En. Carpiaux, Bull. Acad. Eoy Belg., 1908, 

 pp. 283-295) that during incubation some of the shell is dissolved by the 

 carbonic acid formed by the respiration of the chick, and the calcium thus 

 obtained is utilized in growth. The supply of phosphates for bone-formation 

 is obtained from the lecithin of the yolk. 



Why the germ-cell is exposed to such a change of osmotic pressure at all 

 is not clear; but it lias been experimentally proved that dilute solutions are 

 more favourable to cell-division than concentrated ones. It is also possible 

 that this reduction in pressure is a provision for the increase of pressure 

 arising from the metabolism of the chick in the egg, with no possibility of 

 eliminating waste except into the cavity of the allantois ; for the breakdown 

 of the complex proteids of the egg iuto simpler molecules must greatly 

 increase the osmotic pressure ; and this increase might be injuriously great 

 were it not for the previous reduction in pressure. I hope to study the 

 relation further, and to determine the osmotic pressure of the egg during 

 various stages of incubation. 



Summary. 



The blood aud eggs of birds are not isotonic, the osmotic pressure of 

 the egg being considerably the lower. The blood of each kind of bird has 

 an almost constant freezing-point, the fluctuations being of the same order 

 as those met with in mammals. 



The diiierence in the osmotic pressures of the blood and egg is slightly 

 more than accounted for by the diminution in the iuorganic salts of the egg 

 as compared with the blood. 



I wish to thank Dr. Sydney Young, f.r.s., aud Dr. H. H. Dixon, f.r.s., 

 for their advice and criticisms throughout the work, 



