EVIDENCES FROM EMBRYOLOGY 167 
tissues and organs. Almost all the development may take place within 
the egg, so that when the young animal hatches there is necessary little 
more than a rapid growth and increase of size to make it a fully 
developed mature animal. This is the case with the birds; a chicken 
just hatched has most of the tissues and organs of a full-grown fowl], 
and is simply a little hen. But in the case of other animals the young 
hatches from the egg before it has reached such an advanced stage of 
development; a young starfish or young crab or young honeybee just 
hatched looks very different from its parent. It has yet a great deal 
of development to undergo before it reaches the structural condition 
of a fully developed and fully grown starfish or crab or bee. Thus 
the development of some animals is almost wholly embryonic develop- 
ment—that is, development within the egg or in the body of the 
mother—while the development of other animals is largely post- 
embryonic, or larval development, as it is often called. There is no 
important difference between embryonic and postembryonic develop- 
ment. The development is continuous from egg cell to mature animal, 
and whether inside or outside of an egg it goes on regularly and uninter- 
ruptedly. . 
The cells which compose the embryo in the cleavage stage and 
blastoderm stage, and even in the gastrula stage, are apparently 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, develop- 
ment 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, development 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 von Baer, the first great student of 
development. ; —— 
A starfish, 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 structural 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 undifferentiated cells, and then a 
