Page:The Works of William Harvey (part 1 of 2).djvu/547

 chick finds the analogue of both kinds of food in the egg. So that whilst in viviparous animals the uterus exists within the parent, in oviparous the parent may rather be said to exist within the uterus (the egg). For the egg is a kind of exposed and detached uterus, and in it are included in some sort vicarious mammae. The chick in the egg, I say, is first nourished by albumen, but afterwards, when this is consumed, by the yelk or by milk. The umbilical vascular connexion with the albu- men, therefore, when this fluid is used up, withers and is in- terrupted when the abdomen comes to be closed, and be- fore the period of exclusion arrives, so that it leaves no trace of its existence behind it : in viviparous animals, on the con- trary, the umbilical cord is permanent in all its parts up to the moment of birth. The other canal that extends to the vitellus, however, is taken up along with this matter into the abdomen, where being stored, it serves for the support of the delicate foetus until its beak has acquired firmness enough to seize and bruise its food, and its stomach strength sufficient to com- minute and digest it ; just as the young of the viviparous ani- mal lives upon milk from the mammae of its mother, until it is provided with teeth by which it can masticate harder food. For the vitellus is as milk to the chick, as has been already said ; and the bird's egg, as it stands in lieu both of uterus and mammas, is furnished with two fluids of different colours, the white and the yelk.

All admit this distinction of fluids. But I, as I have already said, distinguish two albumens in the egg, kept separate by an interposed membrane, the more external of which embraces the other within it, in the same way as the yelk is surrounded by the albumen in general. I have also insisted on the diverse nature of these albumens ; distinguished both by situation and their surrounding membranes, they seem in like manner calcu- lated for different uses. Both, however, are there for ends of nutrition, the outermost, as that to which the branches of the umbilical veins are earliest distributed, being first con- sumed, and then the inner and thicker portion ; last of all the vitellus is attacked, and by it is the chick nourished, not only till it escapes from the shell but for some time afterwards.

But upon this point we shall have more to say below, when we come to speak of the manner in which the foetuses of