454 ON GENERATION. 



and genital virtue inherent in the egg, expand, and thereby 

 rendered lighter, rise to the top, when the vitellus, with which 

 it is connected follows. It is because the cicatricula, formerly 

 situated on the side of the vitellus, now tends to rise directly 

 upwards that the thicker albumen is made to give place, and 

 the chalazae are carried to the sides of the egg. 



EXERCISE THE SIXTY-FIRST. 



Of the uses of the other parts of the egg. 



The shell is hard and thick that it may serve as a defence 

 against external injury to the fluids and the chick it includes. 

 It is brittle, nevertheless, particularly towards the blunt end, 

 and as the time of the chick's exclusion draws near, doubtless 

 that the birth may suffer no delay. The shell is porous also ; 

 for when an egg, particularly a very recent one, is dressed 

 before the fire, it sweats through its pores. Now these pores 

 are useful for ventilation ; they permit the heat of the incu- 

 bating hen to penetrate more readily, and the chick to have 

 supplies of fresh air ; for that it both breathes and chirps in the 

 egg before its exclusion is most certain. 



The membranes serve to include the fluids, and therefore 

 are they present in the same number as these, and therefore 

 is the colliquament also invested^ as soon as it is produced, with 

 a tunica propria, which Aristotle 1 refers to in these words : " A 

 membrane covered with ramifications of blood-vessels already 

 surrounds the clear liquid," &c. But the exit of the chick 

 being at hand, and the albumen and colliquament being entirely 

 consumed, all the membranes, except that which surrounds the 

 vitellus, are dried up and disappear ; the membrana vitelli, on 

 the contrary, along with the yelk, is retracted into the peri- 

 toneum of the chick and included in the abdomen. Of the 

 membranes two are common to the whole egg, which they sur- 

 round immediately under the shell ; the rest belong, one to the 

 albumen, one to the yelk, one to the colliquament ; but all still 

 conduce to the preservation and separation of the parts they 

 1 Hist. Anim. lib. vi, cap. 3. 



