er of the chief lines of evidence for the truth of the evolution
theory is based on the study of embryology, and this also was followed
with great vigour by the zoologists of the last thirty years of the
nineteenth century. It is found that in many instances animals
recapitulate in their early development the stages through which their
ancestors passed in the course of evolution. Land Vertebrates, including
man, have in their early embryonic life gill-clefts, heart and
circulation, and in some respects skeleton and other organs of the type
found in fishes, and this can only be explained on the assumption that
they are descended from aquatic fish-like ancestors. On the basis of
such facts as these, the theory was formulated that every animal
recapitulates in ontogeny (development) the stages passed through in its
phylogeny (evolution), and great hopes were founded upon this principle
of discovering the systematic position and evolutionary history of
isolated and aberrant forms. In many cases the search has led to
brilliant results, but, as in the case of palaeontology, in many others
the light that was hoped for has not been forthcoming. For it soon
became evident that the majority of animals show adaptation to their
environment not only in their adult stages but also in their larval or
embryonic period, and these adaptations have led to modifications of the
course of development which are often so great as to mask, or obscure
altogether, the ancestral structure which may once have existed.
Although, therefore, the results of embryological research have provided
most convincing proof of the truth of the theory of evolution in
general, they have not completely justified the hopes of the early
embryologists that by this method all the outstanding phylogenetic
problems might be solved.

The detailed study of embryology, however, has led to most important
results apart from the particular purpose for which most of the earlier
investigations in this field were originally undertaken. For the study
of embryology, at first purely descriptive and comparative, was soon
found to involve fundamental problems concerning the factors which
control development. An egg consists of a single cell, and it develops
by the division of this cell into two, then into four, eight, and so
forth, until a mass of cells is produced. In some cases all these cells
are to all appearance alike, or nearly alike; in others the included
yolk is from the fir