DEVELOPMENT OF THE ORGANISM
an embryo before it bursts the egg-shell and begins as a larva to earn its own living. Furthermore, an animal is not hatched in exactly the form of the adult; it attains this form only after a period of growth, and during this period it may properly be termed a larva.
The salamanders show us clearly that the embryonic phase is a secondary modification of the larval phase—in a word, that the embryo is a larva which is provided with shelter and food.
There are two species of European salamander, the black and yellow salamander (Salamandra maculosa), commonly known as the fire salamander, which inhabits the plains and lower reaches of the valleys, and the black salamander (Salamandra atra), which is found in Alpine pastures. Both species bring forth the young alive, but the young of the fire salamander are provided with long, feathery gills and gill slits and pass the first six months of their life in water, whereas the young of the black salamander are devoid of gills and gill slits at birth and are ready to take up the parental habit of life on land.
If now we open the womb of a pregnant black salamander we shall find in it a number of embryos with long, feathery gills, and if these embryos are thrown into water some of them will survive and develop like the young of the fire salamander. The gills are an adaptation to life in water, and when we find them in an embryo we may be sure that the embryo was once a larva and that the larval phase is therefore the primary one.
Similar examples could be adduced from almost every group of animals. We have already alluded to the well-known tadpole larvae of frogs and toads. There is, to mention one more animal, a small West Indian tree frog (Hylodes) which produces a few large eggs from which
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