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 



