226 Mayow 



And hence it is that the said cavity is so much 

 enlarged by incubation. For we must not suppose 

 that this enlargement of the cavity is produced by 

 the access of new air, but by the pressure of the air 

 inside. For although when the blunter end of the 

 egg is perforated, the said cavity is found to be much 

 larger than in new-laid eggs, yet if the sharper end is 

 broken and the juices of the egg poured out, the 

 membrane spread over the cavity (which, by the con- 

 traction of the juices of the egg and by the elastic 

 force of the air, was thrust far into the region of the 

 egg) will immediately fall back, on account of the 

 pressure of the external air introduced into the 

 perforated egg^ and be applied anew to the shell ; so 

 that the cavity will not now appear larger than in 

 unhatched eggs, unless perhaps the air enclosed in it 

 be still warm and rarefied, in consequence of the 

 warmth of the incubating fowl — a clear enough proof 

 that the enlargement of the cavity is due to the 

 elasticity of the air inside, in virtue of which it 

 expands into and occupies the space left by the 

 contraction of the juices. 



Nor yet is it to be supposed that precaution is 

 taken against a vacuum in the egg^ as though nature, 

 according to the common belief, abhorred a vacuum, 

 but rather because a vacuum would not be so suitable 

 for the generation of the chick. And indeed it is 

 reasonable to think that the air stored in the egg^ in 

 consequence of its elastic force (increased not a 

 little by the fostering warmth of the incubating fowl) 

 gently compresses the colliquated juices of the egg 

 and drives them into the umbilical vessels, and hence 

 contributes not a little to the commencement of 

 animal motion. 



It is, moreover, likely that the air inside the egg 



