GENERATION, SEX AND ONTOGENY 229 



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 development that is, development within 

 the egg or in the body of the mother while the development 

 of other animals is largely postembryonic, or larval develop- 

 ment, as it is often called. There is no important difference 

 between embryonic and postembryonic development. The 

 development is continuous from egg cell to mature animal, 

 and whether inside or outside of an egg it goes on regularly 

 and uninterruptedly. 



The cells which compose the embryo in the cleavage stage 

 and blastoderm stage, and even in the gastrula stage, are ap- 

 parently 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 special- 

 ized 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 un- 

 1G 



