TTIE DEVELOPMENT OF THE FROG'S EGG 457 



rise to a particular part of tlie organism. Bj^ repeater! division, 

 a mass of small cells, constituting the morula stage, is formed. 

 Usually a cavity arises in the middle of the morula, and into this 

 some of the surface cells are pushed to form an internal sac — 

 the food canal. This is necessarily an early step, as all food 

 is taken into the interior of the body. The process Ijy which 

 external cells are pushed in is known as gastridation. Very 

 early the body is seen to be composed mostly of la.yers, or 

 membranes and cavities. It is by the folding and union 

 and breaking through of these membranes that most of the 

 organs of the adult arise. Development of the individual is, 

 on the whole, accompanied by increase in complexity. The 

 evolution of animals in the animal kingdom is likewise, on the 

 whole, accompanied by increase in complexity of organization. 

 Thus both the embryonic development of the individual and 

 the evolution of the species proceed from .simple to complex, 

 and since they start from about the same point and reach the 

 same goal, we are not surprised to find that the individual de- 

 velopment of any species often goes through stages markedly 

 like the stages in the evolution of the species. The parallelism 

 of development and evolution was early noticed, and is often 

 called "Von Baer's law," after a naturalist who lived in the 

 middle of the nineteenth century and very clearly formulated 

 this parallelism. 



