Fig. 1.—An egg: a single corpuscle of protoplasm with nucleus b c, and body a.
Accordingly the phases of development or growth of the young are a brief recapitulation of the phases of form through which the ancestors of the young creature have passed. In some animals this recapitulation is more, in others it is less complete. Sometimes the changes are hurried through and disguised, but we find here and there in these histories of growth from the egg most valuable assistance in the attempt to reconstruct the genealogical tree. The history of the development of the common frog is a good illustration of the kind of evidence in question.
Fig. 2.—Tadpoles and young of the Common Frog. 1, Recently hatched (twice natural size); 2 and 2a, same enlarged to show the external gills; 3 and 4, later stages with gill slits covered by a membrane leaving only the spiracle (see Fig. 16) as an exit for the respired water; 5, with hind legs appearing; 6, with both fore and hind legs; 7, atrophy of the tail; 8, young frog.
The frog’s egg first gives rise to a little aquatic creature with external gills and a tail—the tadpole—which gradually loses its gills and its tail and acquires in their place lungs and four legs (Fig. 2), so as now to be fitted for life on dry land. From what we otherwise know of the structure of the frog and the animals to which it is allied, we are justified in concluding that the tadpole is a recapitulative phase of development, and represents to us more or less closely an ancestor of the frog which was provided with gills and tail in the adult state, and possessed neither legs nor lungs.
Fig. 3.—Adult shrimp of the genus Peneus.
A less familiar case is that of a certain kind of shrimp, which is illustrated in the wood-cuts (Fig. 3 and right lower corner of Fig. 4). The little creature which issues from the egg of this shrimp is known as the “Nauplius form.” Many animals very different in appearance from this shrimp make their first appearance in the world as Nauplii; and it appears probable that the Nauplius-phase is the recapitulative re-presentation of an ancestor common to all this set of animals, an ancestor which was not exactly like the Nauplius, but not very different from it.