of the shell renders this impossible in the generality of vertebral animals, and it is not observed to happen in other cases where there is no such obstacle, except indeed it be in fishes, which are said to present a similar peculiarity. In such instances it cannot be supposed that the eggs grow, in the proper sense of that word; they must be considered merely as increasing in volume by the distention of the flexible envelope accommodating itself to the larva, which increases somewhat in size as it assimilates the liquid filling the interior. M. P. Huber found the eggs of ants when ready to be hatched nearly twice the size of those newly laid. Reaumur, however, seems to be of opinion that there is a positive absorption from without of the surrounding fluids, and that in the case of saw-flies and gall-flies, the vegetable juices are imbibed from the leaves on which they are fixed, in a manner which does not easily admit of explanation.
As the period of hatching depends on temperature, it varies with the state of the atmosphere, and the greater or less degree of influence with which that is permitted to act owing to the consistency of the egg-cover. The natural heat too is sometimes modified by the substances in which the eggs are placed, as when the nidus consists of dung, for example. In the heat of summer, the time that elapses between the deposition of the egg and exclusion of the larva is not of long duration; but it is too variable to admit of any general period being mentioned. In perhaps the majority of cases, it varies from one to ten days;
