8 INTRODUCTION. 
other—duplicates in function and details of structure—while a knowl- 
edge of their development may show that they have had entirely 
different origins and different histories, and hence cannot be identical; 
they are examples of what the evolutionist calls convergent evolution. 
in which they occur. Farther, the development affords a framework 
around which the details of organization may be arranged in a logical 
following pages are based on embryology. Not only are the histories 
of the separate organs traced before an account is given of the adult 
form the earlier stages before the organs are outlined. 
r ; The enormously complicated 
2 3 : ; 
body of every vertebrate is derived 
from a comparatively simple special- 
ovum, must be fertilized by a still 
more specialized cell, the spermato- 
Fic. 2.—Successive stages in the seg- this fertilization the egg goes through 
mentation of an amphibian egg. 1-7, 
planes. of changes which bring it contin- 
ually nearer the adult condition. The phases of this differ with 
is subject to modifications in the several groups, for an account of which 
reference must be had to embryological text-books. 
the segmentation or cleavage of the egg, in which it divides again and 
again, until the single-celled egg is converted into a large number of cells 
modified accordingly as the egg is large or small, as it contains varying 
amounts of nourishment—deutoplasm or food yolk stored up for the 
development; the description given here follows the simplest conditions. 
As a result of segmentation the egg is converted into a spherical 
cavity because it is formed during segmentation. It also has the 
name archiceele as it is the first or oldest space to appear in the 
Such cases are apt to lead one astray as to the relations of the forms 
manner, thus aiding in their remembrance. For these reasons the 
conditions, but this introductory chapter gives in the most generalized 
ized cell, the egg or ovum. This 
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ce zoon, derived from the male. After 
Results of the corresponding cleavage an orderly but very gradual series 
different animals; here only a generalized account will be given, which 
The Segmentation of the Egg.—The first steps of the process are 
or blastomeres (fig. 2). The character of this segmentation is 
growing embryo. These same variations also affect the later stages of 
mass of cells in which a cavity appears, called the segmentation 
embryo. This stage of the embryo is called the blastula (fig. 3). 
