172 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii 



used by Mazin. His fundamental mistake here was that he failed to 

 realise that the egg-shell was permeable to air; and this vitiates all 

 his reasoning about the respiration of the egg. "It will not be ir- 

 relevant", he says, "to enquire here whether the air which is con- 

 tained in the cavity in the blunter end of every egg contributes to 

 the respiration of the chick." He first notes that the cavity in question 

 lies between two membranes and not between the shell-membrane 

 and the shell as Harvey himself had supposed ; and then he goes on 

 to say that he disagrees with the opinion of Fabricius, who had asserted 

 that the air in the air-space serves for the respiration of the chick. 

 His reasons are (i) that there would not be enough therein for the 

 needs of the embryo which would use it, as it were, in one gulp, 

 and (2) that the air in it cannot pass through the inner membrane, 

 an error into which he was led by observing that, if an egg-shell 

 with its contents removed and its air-space intact, was put into a 

 vacuum, the air-space would swell up until it was as big as the egg 

 itself. Mayow sees now what had escaped the attention of all previous 

 observers, namely, that the egg-contents are not "rarefied or ex- 

 panded, but are on the contrary condensed and forced into a nar- 

 rower space than before". Such a condensation could, he thinks, 

 take place in four ways, (a) by an increase in propinquity of discrete 

 particles, (b) by a subsidence of motion on the part of a congregation 

 of particles into rest, (c) by the extraction of some subtle spirit from 

 amongst the particles, and, (d) by a decrease in elasticity on the part 

 of some elastic substance previously present. We should at the present 

 time choose the third alternative as being the truest, in view of the 

 loss of water and carbon dioxide which the egg suffers as it develops, 

 but Mayow chose the fourth, thinking it probable that the "air 

 distributed among the juices of the egg loses its elastic force on account 

 of the fermentation produced among these juices by incubation". 

 Now since the egg-contents are compacted into smaller bulk by the 

 process of incubation, a vacuum would be created somewhere if 

 Nature had not, with her customary prudence, inserted a small 

 amount of air into the air-space which might in due course expand 

 and avoid this. His proof for this was an inaccurate observation; 

 he thought he saw, in eggs at a late stage, when the contents were 

 removed, the air-space collapse to the normal size which it occupies 

 in unincubated eggs. He expressly says that his theory does not depend 

 upon the conception of horror vacui, but that, by the compressive 



