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Abstract:
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From the introduction: The purpose of the following work from its inception has
been to make as careful a study as possible of the cleavage of
the ovum, the formation of the germinal layers and definitive
organs, and the axial relations of the ovum to the larval and
adult axes. At the time when this work was begun, several
years ago, scarcely any attempts had been made to trace the
history of individual blastomeres through the entire development
to the formation of definitive organs. The early stages
of cleavage had received a great deal of attention, but the
later stages had been largely neglected; and although the
origin and homology of the germ layers was perhaps the most
frequently discussed subject in embryology, yet the relation of
these layers to the individual blastomeres of the cleaving ovum
had been determined in comparatively few cases. Since that time a number of very valuable papers have appeared on this
subject of “cell lineage,” as Wilson (‘92) has aptly termed it.
The results of such work are no longer as novel as they were
four or five years ago, and yet the general interest in the
subject has greatly increased, and that, too, in spite of the fact
that there is a growing school of biologists who believe that
individual blastomeres have no necessary relation to future
organs. The subject of germ layers is no longer so important
as it was once considered; in fact, the theory of the homology
of the germinal layers has met with so many difficulties of late
that it is now generally maintained only in a greatly modified
form. However, the fundamental idea which was prominent in
germ-layer discussions is of vital interest to-day. In the whole
history of the germ-layer theories I see an attempt to trace
homologies back to their earliest beginnings. This problem
is as important to-day as it ever was, and whether one find
these earliest homologies in layers or regions or blastomeres
or the unsegmented ovum itself, the quest is essentially the
same. |