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 continuous 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 first 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 



