The Works of William Harvey M.D. / Translated from the Latin with a life of the author

by William Harvey

EXERCISE THE SIXTEENTH.

Second inspection of the egg.

The second day gone by, the circles of the cicatricula that have been mentioned, have become larger and more conspicuous, and may now be of the size of the nail of the ring-finger, sometimes even of that of the middle finger. By these rings the whole cicatricula is indistinctly divided into two, occasionally into three regions, which are frequently of different colours, and bear a strong resemblance to the cornea of the eye, both as respects dimensions, a certain degree of prominence, and the presence of a transparent and limpid fluid included within it. The centre of the cicatricula here stands for the pupil; but it is occupied with a certain white speck, and appears like the pupil of some small bird’s eye obscured by a suffusion or cataract, as it is called. On this account we have called the entire object the oculum ovi, the eye of the egg.

Within the circles of the cicatricula, I say, there is contained a quantity of perfectly bright and transparent fluid, even purer than any crystalline humour; which, if it be viewed transversely and against the light, the whole spot will rather appear to be situated in the albumen than sunk into the membrane of the yelk, as before: it presents itself as a portion of the albumen dissolved and clarified, and included within a most delicate tunica propria. Hence I entitle this fluid the oculum seu colliquamentum album; it is as if a portion of the albumen, liquefied by the heat, shone apart, (which it does, unless disturbed by being shaken,) and formed a more spirituous and better digested fluid, separated from the rest of the albumen by a tunica propria, and situated between the two masses of liquid, the yelk and the albumen. It differs from the rest of the albumen by its clearness and transparency, as the water of a pellucid spring differs from that of a stagnant pool. The tunic which surrounds this fluid is so fragile and delicate that, unless the egg be handled with great care, it is apt to give way, when the pure spring is rendered turbid by a mixture of fluids.

I was long in doubt what I should conclude as to this clear diffluent fluid, whether I should regard it as the innate heat, or radical moisture; as a matter prepared for the future fœtus, or a perfectly-concocted nourishment, such as dew is held to be among the secondary humours. For it is certain, as shall be afterwards shown, that the earliest rudiments of the fœtus are cast in its middle, that from this the chick derives its first nutriment, and even when of larger size continues to live amidst it.

This solution therefore increases rapidly in quantity, particularly in its internal region, which, as it expands, forces out and obliterates the external regions. This change is effected in the course of a single day, as is shown in the second figure of Fabricius. It is very much as it is with the eyes of those animals which have a very ample pupil, and see better by night than by day, such as owls, cats, and others, whose pupils expand very much in the dusk and dark, and, on the contrary, contract excessively in a brilliant light: one of these animals being taken quickly from a light into a shady place, the pupil is seen to enlarge in such wise that the coloured ring, called the iris, is very much diminished in size, and indeed almost entirely disappears.

Parisanus, falling upon these regions, is grossly mistaken when he speaks of “a honey-coloured, a white, a gray, and another white circle;” and says that “the fœtus is formed from the white middle point” (which, indeed, appears in these regions), and that “this is the semen of the cock.” That he may exalt himself on a more notable subtlety he continues: “Before any redness is apparent in the body of the fœtus, two minute vesicles present themselves in it; in the beginning, however, neither of them is tinged with red;” one of these he would have us receive as the heart, the other as the liver. But in truth there is neither any vesicle present sooner than the redness of the blood is disclosed; nor does the embryo ever suddenly become red in the course of the first days of its existence; nor yet does any of these vesicles present us with a trace of the liver. Both of them belong, in fact, to the heart, prefiguring its ventricles and auricles, and palpitating, as we shall afterwards show, they respond reciprocally by their systoles and diastoles.

Aristotle[175] appears to have known this dissolved fluid, when he says: “A membrane, too, marked with sanguineous fibres, surrounds the white fluid at this time (the third day), arising from those orifices of the veins.” Now the philosopher can neither be supposed by the words “white fluid,” to refer to the albumen at large, because at this period the membrane of the white is not yet covered with veins; it is only the membrane of the dissolved fluid which appears with a few branches of veins distributed over it here and there. And because he says: “this membrane, too,” as if he understood another than those which he had spoken of as investing the albumen and the yelk before incubation, and designated this one as first arising after the third day, and from the orifices of the veins.

Coiter seems also to have known of this dissolved fluid; he says: “A certain portion of the albumen acquiring a white colour, another becoming thicker.” The fluid in question is surrounded with its proper membrane, and is distinct and separate from the rest of the albumen before there is any appearance of blood. We shall have occasion, by and by, to speak of the singular importance of this fluid to the fœtuses of every animal. Whilst they float in it they are safe from succussion and contusion, and other external injury of every kind; and they moreover are nourished by it. I once showed to their serene majesties the king and queen, an embryo, the size of a French-bean, which had been taken from the uterus of a doe; all its membranes were entire, and from its genital organs we could readily tell that it was a male. It was, in truth, a most agreeable natural spectacle; the embryo perfect and elegant, floating in this pure, transparent, and crystalline fluid, invested with its pellucid tunica propria, as if in a glass vessel of the greatest purity, of the size of a pigeon’s egg.