Page:The Works of William Harvey (part 1 of 2).djvu/554

 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.