84 ANIMAL LIFE 
wholly embryonic development—that is, development with- 
in the egg or in the body of the mother—while the devel- 
opment of other animals is largely post-embryonic or larval 
development, as it is often called. There is no important 
difference between embryonic and post-embryonic develop- 
ment. The development is continucus from egg cell to 
mature animal, and whether inside or outside of an egg it 
goes on regularly and uninterruptedly. 
52. Development after the gastrula stage.—The cells which 
compose the embryo in the cleavage stage and blastoderm 
stage, and even in the gastrula stage, are all similar; there 
is little or no differentiation shown among them. But from 
the gastrula stage on development includes three important 
things: the gradual differentiation of cells into various 
kinds to form the various kinds of animal tissues; the 
arrangement and grouping of these cells into organs and 
body parts; and finally the developing of these organs 
and body parts into the special condition characteristic of 
the species of animal to which the developing individual 
belongs. From the primitive undifferentiated cells of the 
blastoderm, development leads to the special cell types of 
muscle tissue, of bone tissue, of nerve tissue; and from the 
generalized condition of the embryo in its early stages de- 
velopment leads to the specialized condition of the body of 
the adult animal. Development is from the general to the 
special, as was said years ago by the &rst great student of 
development. 
53. Divergence of development.—A star-fish, a beetle, a 
dove, and a horse are all alike in their beginning-—that is, 
the body of each is composed of a single cell, a single struc- 
tural unit. And they are all alike, or very much alike, 
through several stages of development; the body of each 
is first a single cell, then a number of similar undifferen- 
tiated cells, and then a hollow sphere consisting of a single 
layer of similar undifferentiated cells. But soon in the 
course of development the embryos begin to differ, and as 
