ovum, is provided in every class. Only in those animals possessing soft and distensible bodies, can the whole or greater part of this material become ripened and brought to perfection; and even here, where the ripened ova amount sometimes to mil- lions, the process must be gradual; for, after the oviposit has taken place, there may still be seen, within the ovisacs, the preparations for those which are to follow in the next breeding season. But as we ascend higher in the scale, and come to animals producing a smaller progeny at one time, or even to those that are commonly uniparient, the difference consists not so much in a proportionate diminution of the number of germs or ova originally formed as of those that ultimately come to perfection; for, in some of the Mammalia at least, it is probably calculating below the mark if we say that many hundred original germs are provided to meet the contingency of a single act of birth. Of these, therefore, the greater part can never be ripened at all, but must perish in the seat of their original formation (g); and, further, if those which are ripened and emitted do not become fertilized shortly after that event, these equally perish.
In order to trace out this matter further, we
