26o THE UNFERTILISED EGG [pt. m 



permeability of the shell and its membranes will also be dealt with. 

 There has been some controversy on the subject of whether the egg- 

 shell contains any elements of the secreting organ in it, like the decidua 

 of mammals. Von Baer thought not, but the presence of cellular 

 structures has been reported by von Hemsbach; Landois; and 

 Blasius. 



The shell-membrane has been studied chemically by Liebermann 

 and Lindwall, who found that it consisted almost entirely of a protein, 

 the percentage composition of which agreed very closely with keratin 

 (Table loa). Krukenberg, alone, on the ground of its reactions, held 

 it to be a mucin. This ovokeratin, which contains four times as much 

 sulphur as the albumen of the egg-white, was found by Morner to 

 include 7 per cent, of cystine, but there are reasons for supposing 

 that this figure is much too low. Nothing is known of the part 

 played by ovokeratin in embryonic metabolism, but, in view of the 

 fact that calcium is transported from the shell to the embryo during 

 the period of ossification of embryonic cartilage, it is not impossible 

 that the sulphur or the cystine of ovokeratin may be made use of 

 in a similar manner to meet the need for sulphur and cystine 

 for the feathers. This will be discussed later under the head of 

 sulphur metabolism (Section I2'7). Morner considered that sulphur 

 must exist in the ovokeratin in other forms besides cystine, for that 

 amino-acid would not account for more than a third of what he found 

 was there. The amino-acid analyses of ovokeratin are placed in 

 Table 1 1 ; they are due to Abderhalden & Ebstein, and to Plimmer & 

 Rosedale, the former by isolation and the latter by the van Slyke 

 nitrogen distribution method. The arginine figure is rather high. 



The strength of the shell is clearly an important biological factor : 

 according to Romanov its average thickness is 0*3 11 mm. giving a 

 breaking-strength of 4-46 kilos. The relation between shell-thickness 

 and breaking-strength is a straight line. 



The physiological properties of the shell of the bird's egg have been 

 very insufficiently studied. In the last century there was a general 

 impression that the shell possessed a differential permeability and 

 that, while water and other liquids would readily go through the 

 egg-shell and its membranes from outside in, they would not easily 

 pass from the inside to the outside. It is difficult to find how this 

 idea originated; thus Ranke in 1872 attributed it to the younger 

 Meckel, and Ranke's own statement was subsequently copied down 



