336 ZOOLOGY 



But the ripe egg of many of the lower plants and animals 

 requires no fertilization for development, and the meaning 

 of the fertilization process is quite obscure. 



The developing eggs of all the higher animals pass 

 through much the same sort of early stages. The egg 

 " cleaves " into a number of cleavage spheres, each of 

 which is destined to give rise to a particular part of the 

 organism. By repeated 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 pro- 

 cess by which external cells are pushed in is known as 

 gastrulation. Very early the body is seen to be composed 

 mostly of layers, or membranes and cavities. It is by the 

 folding and union and breaking through of these mem- 

 branes that most of the organs of the adult arise. Devel- 

 opment 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 evolu- 

 tion 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 

 development 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 cen- 

 tury and very clearly formulated this parallelism. 



