SECT. l] 



THE UNFERTILISED EGG 



257 



birds, and it would be interesting, for example, to know why the 

 pheasant's contains such an unusually high percentage, of phos- 

 phorus, and why the herring gull's has so high a percentage of organic 

 substance. A certain interest attaches to the determinations of 

 Balland on ostrich eggs, some from a tomb 

 of the Hellenistic period and others from 

 modern ostriches, but the differences he 

 found were probably not very significant, 

 as the analysis of Torrance seems to give 

 values half-way between those of Balland. 

 Neither Balland, Torrance nor Wicke states 

 whether the ostrich used was the North 

 or South African variety, a complication 

 which might make a difference. Wicke 

 believed that the difference in shell-com- 

 position between different kinds of birds 

 was almost entirely dependent on their 

 usual foods. 



The microscopic structure of the shell 

 was investigated by Nathusius in the 'sixties, 

 and since then little has been added to his 

 work. The shell consists of an outer layer of 

 crystals of calcium carbonate arranged with 

 their long axes perpendicular to the bound- 

 ing surface (Fig. 16), and an inner layer 

 composed of undifferentiated calcium car- 

 bonate (Herzog & Gonell). Kelly; Schmidt; 

 Meigen and Osawa have found that the 

 mineralogical form of the lime is invariably 

 calcite, no aragonite being present in any 

 bird's egg-shell. This has been confirmed 

 with X-ray analysis by Mayneord. The 

 ostrich, Emj>s europaea, is the only doubtful case, for Kelly identified 

 its egg-shell lime as conchite, but Torrance considers it to be calcite. 

 Prenant's review should be consulted for further details regarding 

 this interesting biochemical problem. Only one paper exists dealing 

 with the changes which the shell undergoes histologically during the 

 development of the chick; it will be considered in the section on em- 

 bryonic respiration, where the data we possess on the question of the 

 NEi 17 



-^:- -;>.,«■ j 



Fig. 16. a, Outer crystalline 

 layer; b, c, d, amorphous 

 layers; e, mamillae; /, shell- 

 membrane. 



