RESPIRATION IN THE EMBRYO AND FCETUS 560 



Respiration in the Embryo and Foetus. The processes of 

 respiration in the embryo have been the subject of numerous in- 

 vestigations upon the eggs of birds, reptiles, amphibians and fishes. 

 As early as 1674 Mayow had recognised that the nitro-aerial gas, 

 that is oxygen, was absorbed through the porous shell of a hen's 

 egg during incubation. Blumenbach observed that the blood 

 in the allantoid veins was brighter in colour than that in the 

 allantoid arteries, and Paris found that the air in the air-chamber 

 pf the egg contained carbon dioxide during the last days of in- 

 cubation. 



The necessity of air for all stages of the development of the 

 embryo has been proved by varnishing the egg or covering it with 

 oil to prevent the passage of gases ; development does not occur, 

 or if the experiment be performed at a later stage of incubation 

 death is rapidly caused. In a few experiments development has 

 proceeded in the early stages, but these cases are due to defective 

 varnish, development before the experiment commenced, or with 

 the aid of the oxygen already present in the egg. It is interesting 

 to note that Hasselbach maintains that a small quantity of oxygen 

 may be set free by chemical changes occurring in the substance 

 of the egg at the commencement of incubation. Eggs cannot be 

 developed by incubation in an atmosphere of pure hydrogen or 

 nitrogen. 



The air-chamber at the blunt end of the hen's egg is present 

 whether the egg be fertile or not, and is enlarged during the period 

 of incubation by the evaporation of water. It serves as a reservoir 

 of air for the embryo and even for the chick during incubation, 

 for it is well known that the chick may, after pushing its beak 

 through the membranes, breathe the air in this space by its lungs 

 for some hours before it breaks the shell. The chick often draws 

 its first breath and even chirps before it is hatched. 



The total respiratory exchange can be easily determined in 

 the developing chick. There is an absorption of oxygen and a 

 production of carbon dioxide during the first hours of incubation, 

 and this gaseous exchange steadily increases with the process of 

 development ; relatively it is equal to, or even greater than, that 

 of the adult hen. It has already been mentioned that the 

 respiratory exchange of the embryo chick is affected by changes 

 of temperature in a similar manner to that seen in cold-blooded 

 animals, a fall of temperature causes a decrease, a rise of tempera- 



