This historic book may have numerous typos and missing text.
Purchasers can usually download a free scanned copy of the original
book (without typos) from the publisher. Not indexed. Not
illustrated. 1914 edition. Excerpt: ... organism. There appears to
be every gradation in the degree of such differentiation. In some
cases there is apparently none at all. In others, different zones
or strata may be seen. In many such eggs mutilation of certain
parts of the egg-cell produces corresponding defects in the
resultant organism, showing that the protoplasm was already
specifically differentiated before cleavage. The amount and
distribution of the food-yolk is of the greatest influence in
determining the nature of the cleavage process. In an isolecithal
egg the first cleavage usually divides the egg-cell into two
approximately equal daughter-cells, which, in turn, divide, each
into two others of about the same size. This process is repeated in
rapid succession until the original zygote has been halved,
quartered, and then cut into 8, 16, 32, 64, 128 cells, etc., the
process continuing until the first step in differentiation occurs.
The result of such repeated cell division is a rounded mass of tiny
cells that looks somewhat like a mulberry, and, for that reason,
the stage is often called the morula. In very few species, however,
is the process of cleavage so regular and symmetrical as just
described. In many forms, not only are the cells of a different
size, but the cleavages do not all occur in regular order, some
coming on faster than others. In nearly all cases, however, the
result of the continued cleavage is to produce a mass of cells that
remain attached to one another in a single layer. Owing perhaps to
the stress of surface tension acting on the cells and pulling them
away from the center, the resultant is a hollow sphere. The stage
just described is called the blastula. The closed cavity surrounded
by the single layer of cells (blastomeres) is called the...
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