ON GENERATION, 459 



there also be several species of eggs ; for every primordium is 

 not alike fit to receive or assume every variety of animal form 

 indifferently. Though we admit, therefore, that eggs in a gene- 

 ral sense do not differ, yet when we find that one is perfect, 

 another imperfect, it is obvious that they differ essentially from 

 one another. Perfect eggs are such as are completed in the 

 uterus, where they obtain their due dimensions before being 

 extruded ; of this kind are the eggs of birds. Imperfect eggs, 

 again, are such as are prematurely excluded before they are of 

 the full size, but increase after they are laid ; of this descrip- 

 tion are the eggs of fishes, Crustacea and mollusca ; the primor- 

 dia of insects, which Aristotle entitles worms, are farther to be 

 referred to this class, as well as the primordia of those animals 

 that arise spontaneously. 



Moreover, although perfect eggs are of two colours, in other 

 words, are composed of albumen and vitellus, some are still 

 only of one hue, and consist of albumen alone. In like 

 manner, of imperfect eggs, some from which a perfect animal 

 proceeds are properly so called; such are the eggs of fishes; 

 others are improperly so styled, they engendering an imperfect 

 animal, namely, a worm, grub, or caterpillar, a kind of mean 

 between a perfect and an imperfect egg, which, in respect of 

 the egg or the primordium itself, is an animal endowed with 

 sense and motion, and nourishing itself; but in respect of a 

 fly, moth or butterfly, whose primordium it is potentially, it is 

 as a creeping egg, and to be reputed as adequate to its own 

 growth ; of this description is the caterpillar, which having at 

 length completed its growth is changed into a chrysalis or 

 perfect egg, and ceasing from motion, it is like an egg, an 

 animal potentially. 



In the same way, although there are some eggs from the 

 whole of which a perfect animal is produced by metamorphosis, 

 without being nourished by any remains of the substance of 

 the egg, but forthwith finds food for itself abroad, there are 

 others from one part of which the embryo is produced, and from 

 the* remainder of which it is nourished: although,! repeat, there 

 are such differences among eggs, still, if we be permitted to con- 

 clude on the grounds of sense and analogy, there is no good 

 reason wherefore those that Aristotle calls worms should not 

 be spoken of as eggs ; inasmuch as all vegetal principles are 



