Thus it comes to pass that every one, in adducing reasons for the formation of the chick in ovo, in accordance with preconceived opinions, has wandered from the truth. Some will have it that the semen or the blood is the matter whence the chick is engendered; others, that the semen is the agent or efficient cause of its formation. Yet to him who dispassionately views the question is it quite certain that there is no prepared matter present, nor any menstruous blood to be coagulated at the time of intercourse by the semen masculinum, as Aristotle will have it; neither does the chick originate in the egg from the seed of the male, nor from that of the female, nor from the two commingled. EXERCISE THE FIFTEENTH. _The first examination of the egg; or of the effect of the first day’s incubation upon the egg._ That we may be the more clearly informed of the effect which the first day’s incubation produces upon the egg, we must set out by ascertaining what changes take place in an egg spontaneously, changes that distinguish a stale egg from one that is new-laid, when what is due to the incubation _per se_ will first be clearly apprehended. The space or cavity in the blunt end is present, as we have said, in every egg; but the staler the egg the larger does this hollow continually grow; and this is more especially the case when eggs are kept in a warm place, or when the weather is hot; the effect being due to the exhalation of a certain portion of the thinner albumen, as has been stated in the history of the egg. This cavity, as it increases, extends rather in the line of the length than of the breadth of the egg, and comes finally to be no longer orbicular. The shell, already less transparent, becomes dingy. The albumen grows thicker and more viscid, and acquires a straw or yellow colour. The tunica propria of the vitellus becomes more lax, and appears wrinkled, for it seems that some even of this fluid is dissipated in the course of time.