722 



THE RESPIRATION AND 



[PT. Ill 



have been made of the shell during the development of the chick, but 

 analyses which will be discussed later (see Section 13-2) do demonstrate 

 that there is a definite loss of inorganic and organic substances from 

 the shell, and it is a well-known fact that the shell becomes more 

 brittle as development proceeds. Only one study of the histology of 

 the shell during incubation has been undertaken, namely, that of 

 Rizzo in 1899. Rizzo found that the number of pores per square 

 millimetre of shell surface varied from o-86 to 1-44, with an average 

 of 1-23. A hen's egg has an average surface of 6644 sq. mm., and about 

 7600 pores. Rizzo's method was to drain the contents of the egg 



Ferbi 



e eggs 



COaCold'Olen 



OzCold'Olen 

 02Norm3l 



through two small punctures, after which the egg-shells were carefully 

 washed, and refilled with a weak aqueous solution of methylene blue 

 — the pores were then visible to the naked eye as fine blue points. 

 They were much more numerous over the air-space than elsewhere^. 

 We may conclude that an increased permeability of the shell to 

 carbon dioxide during development is fairly well established and 

 the remarks that have been made on this point apply equally well 

 to oxygen. The loss in oxygen is more or less compensated for by the 

 gain in carbon dioxide, so that the nitrogen content of the air-space 

 remains practically unchanged. Hufner, whose permeability experi- 

 ments on egg-shells have already been referred to, found that in 

 I second at 11-9° 2-115 c.c. of oxygen would diffuse into the goose's 

 egg (from ordinary air, i.e. a partial pressure of 159 mm.) and 

 0-503 c.c. of carbon dioxide would diffuse out (to a partial pressure 



^ For a physical account of these pores, see Dumanski & Strukova. 



