generation a judgment may be formed of that of all other viviparous animals. Wherefore I shall propose a single genus, by way of general example or type, as we did in the case of the oviparous class ; this made familiar to us, will serve as a light or standard, by means of which all the others may be judged of by analogy.
The reasons that led me to select the hen's egg as the mea- sure of eggs in general have been already given : eggs are of little price, and are everywhere to be obtained, conditions that permit repeated study, and enable us cheaply and readily to test the truth of statements made by others.
We have not the same facilities in studying the generation of viviparous animals : we have rarely, if ever, an opportunity of dissecting the human uterus ; and then to enter on the subject experimentally in the horse, ox, sheep, goat, and other cattle, would be attended with immense labour and no small expense; dogs, cats rabbits, and the like, however, will supply those with subjects who are desirous of putting to the test of experiment the matters that are to be delivered by us in this place.
Fabricius of Aquapendente, as if every conception of a vivi- parous animal were in a certain sense an egg, begins his trea- tise with the egg as the universal example of generation ; and among other reasons for his conclusions assigns this in parti- cular : l " Because the study of the egg has the most exten- sive application, the greater number of animals being engen- dered from eggs." Now we, at the very outset of our observa- tions, asserted that ALL animals were in some sort produced from eggs. For even on the same grounds, and in the same manner and order in which a chick is engendered and deve- loped from an egg, is the embryo of viviparous animals engendered from a pre-existing conception. Generation in both is one and identical in kind : the origin of either is from an egg, or at least from something that by analogy is held to be so. An egg is, as already said, a conception ex- posed beyond the body of the parent, whence the embryo is produced; a conception is an egg remaining within the body of the parent until the foetus has acquired the requisite perfec-
1 De Form. Ovi et Pulli, cap. 1.
