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. 

 Such cases are apt to lead one astray as to the relations of the forms 

 in which they occur. Farther, the development affords a framework 

 around which the details of organization may be arranged in a logical 

 manner, thus aiding in their remembrance. For these reasons the 

 following pages are based on embryology. Not only are the histories 

 of the separate organs traced before an account is given of the adult 

 conditions, but this introductory chapter gives in the most generalized 

 form the earlier stages before the organs are outlined. 



The enormously complicated 

 body of every vertebrate is derived 

 from a comparatively simple special- 

 ized cell, the egg or ovum. This 

 ovum, must be fertilized by a still 

 more specialized cell, the spermato- 

 zoon, derived from the male. After 

 FIG. 2. Successive stages in the seg- this fertilization the egg goes through 

 Srofthe an co a ^plndr ng eg ea47e an orderly but very gradual series 

 planes, of changes which bring it contin- 



ually nearer the adult condition. The phases of this differ with 

 different animals; here only a generalized account will be given, which 

 is subject to modifications in the several groups, for an account of which 

 reference must be had to embryological text-books. 



The Segmentation of the Egg. The first steps of the process are 

 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 

 or blastomeres (fig. 2). The character of this segmentation is 

 modified accordingly as the egg is large or small, as it contains varying 

 amounts of nourishment deutoplasm or food yolk stored up for the 

 growing embryo. These same variations also affect the later stages of 

 development; the description given here follows the simplest conditions. 

 As a result of segmentation the egg is converted into a spherical 

 mass of cells in which a cavity appears, called the segmentation 

 cavity because it is formed during segmentation. It also has the 

 name archiccele as it is the first or oldest space to appear in the 

 embryo. This stage of the embryo is called the blastula (fig. 3). 



