form. I regard all those that diverge from the primary palingenetic
gastrulation as secondary, modified, and cenogenetic. The more or less
divergent form of gastrula that is produced may be called a secondary,
modified gastrula, or a metagastrula. The reader will find a scheme of
these different kinds of segmentation and gastrulation at the close of
this chapter.

By far the most important process that determines the various
cenogenetic forms of gastrulation is the change in the nutrition of
the ovum and the accumulation in it of nutritive yelk. By this we
understand various chemical substances (chiefly granules of albumin
and fat-particles) which serve exclusively as reserve-matter or food
for the embryo. As the metazoic embryo in its earlier stages of
development is not yet able to obtain its food and so build up the
frame, the necessary material has to be stored up in the ovum. Hence
we distinguish in the ova two chief elements--the active formative
yelk (protoplasm) and the passive food-yelk (deutoplasm, wrongly
spoken of as "the yelk"). In the little palingenetic ova, the
segmentation of which we have already considered, the yelk-granules
are so small and so regularly distributed in the protoplasm of the
ovum that the even and repeated cleavage is not affected by them. But
in the great majority of the animal ova the food-yelk is more or less
considerable, and is stored in a certain part of the ovum, so that
even in the unfertilised ovum the "granary" can clearly be
distinguished from the formative plasm. As a rule, the formative-yelk
(with the germinal vesicle) then usually gathers at one pole and the
food-yelk at the other. The first is the ANIMAL, and the second the
VEGETAL, pole of the vertical axis of the ovum.

(FIGURE 1.39. Gastrula of the amphioxus, seen from left side