which their parents passed; after that, and then only, does the effect of the black environment begin to tell on them, for the yellow spots begin to diminish in size.[1]
The young forms termed larvae, which by their structure and habits repeat ancestral conditions, live freely in the world and earn their own living. The most familiar example of a larva is the tadpole of the frog, which, by its gills and tail recalls the fish-like ancestors of the frogs. There is another type of young animal which is known as an embryo (Greek εν (εμ), in, βρύειν, grow). This type, during its development, is sheltered and fed either within an egg shell or in the womb of the mother. A good example of this type is the chicken, of which the greater part of the development is completed within the egg shell. The young form in this type derives its food from the yolk in the egg, which it slowly digests as it grows, and from the “white,” or albumen. Another variety of embryo is sheltered within the womb of the mother and obtains all its nourishment from the maternal blood. The embryos of all the higher warm-blooded mammals, such as those of dogs, horses, and cattle, as well as the human embryo, are of this kind.
Now let us consider how the embryonic and the larval types of development are related to one another. Was the original form of development embryonic or larval?
When we closely examine the life histories of animals we discover that there is an embryonic and a larval phase in all, though these phases are of extremely different lengths in different animals. No animal deposits naked eggs; an egg-shell, though it may be thin and elastic, is always formed, and the egg always includes some yolk, so that there is always a period during which the young animal develops as
- ↑ All biologists are not agreed as to the sufficiency of the evidence for the inheritance of acquired characters.—Ed.
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