Kowalevsky.) (Front view.))

In view of this extraordinary significance of the gastrula, we must
make a very careful study of its original structure. As a rule, the
typical gastrula is very small, being invisible to the naked eye, or
at the most only visible as a fine point under very favourable
conditions, and measuring generally 1/500 to 1/250 of an inch (less
frequently 1/50 inch, or even more) in diameter. In shape it is
usually like a roundish drinking-cup. Sometimes it is rather oval, at
other times more ellipsoid or spindle-shaped; in some cases it is half
round, or even almost round, and in others lengthened out, or almost
cylindrical.

I give the name of primitive gut (progaster) and primitive mouth
(prostoma) to the internal cavity of the gastrula-body and its
opening; because this cavity is the first rudiment of the digestive
cavity of the organism, and the opening originally served to take food
into it. Naturally, the primitive gut and mouth change very
considerably afterwards in the various classes of animals. In most of
the cnidaria and many of the annelids (worm-like animals) they remain
unchanged throughout life. But in most of the higher animals, and so
in the vertebrates, only the larger central part of the later
alimentary canal develops from the primitive gut; the later mouth is a
fresh development, the primitive mouth disappearing or changing into
the anus. We must therefore distinguish carefully between the
primitive gut and mouth of the gastrula and the later alimentary canal
and mouth of the fully developed vertebrate.* (* My distinction (1872)
between the primitive gut and mouth and the later permanent stomach
(metagaster) and mouth (metastoma) has been much criticised; but it is
as much justified as the distinction between the primitive kidneys and