112 HABITS OF BIRDS. 



separated after the egg is laid, though they may be 

 found loose and perfectly distinct at the period of 

 incubation. It is very remarkable, and the fact has 

 long been known, that the chalazes and the germ 

 (cicatriculd) always observe determinate relative 

 positions, the germ being uniformly situated at the 

 equator of the egg, while the chalazes occupy 

 the poles, or rather a place very near the poles, for 

 they divide the yolk into two parts of unequal size, 

 and are not always placed in the direction of its axis. 

 The portion opposite the germ being the heaviest, of 

 course always tends to occupy the lowest station, so 

 that the germ being always uppermost, is disposed 

 in the most favourable manner for receiving the in- 

 fluence of the heat in hatching. Harvey's explana- 

 tion of this was, that the chalazes served, as it were, 

 for the poles of the egg and the connections of all 

 the membranes twisted and knit together, by which 

 the liquors are not only conserved each in its place, 

 but also retain their due position to one another*. 

 But this, as Derham well remarks, was short of the 

 facts ; the chalazes " serve not barely to keep the 

 liquors in their place and position to one another, 

 but also to keep one and the same part of the yolk 

 uppermost, let the egg be turned nearly which way 

 it will ; which is done by this mechanism : the cha- 

 laza? are specifically lighter than the whites in which 

 they swim ; and being braced to the membrane of 

 the yolk, not exactly in the axis of the yolk, but 

 somewhat out of it, causeth one side of the yolk to be 

 heavier than the other ; so that the yolk being by the 

 chalazre made buoyant and kept swimming in the 

 midst of two whites, is by its own heavy side kept 

 with the same side always uppermost t." It is on 

 this account, as Willughby remarks, that it is nearly 



* Exercit. de Gener. p. 13. 

 f Physicu-Theology, B. vii. c. 4. n. 6. 



