386 THEORIES OF DEVELOPMENT 



it. The mesoderm gives rise to the bones and muscles, the cir- 

 culatory system, and the muscular walls of the alimentary tube. 

 All metazoans pass through these early stages of development, 

 and embryology teaches that from these simple beginnings the 

 most complex animal body is developed. 



This sphere of cells is known as the morula or mulberry stage 

 (Fig. 5), and is succeeded by the l)lastula stage, in which the 

 cells are arranged in a circle about a cavity filled with watery 

 fluid. On account of inequalities, one portion of this sphere 

 becomes pitted, this pitting-in grows deeper until there is a 

 complete invagination of this portion, like the pushing in of one 

 side of a hollow rubber ball until the two sides touch. This is 

 called the gastrula stage, and occurs in all the main divisions 

 of the animal kingdom. (In many vertebrates delamination 

 supercedes or follows invagination, but this, perhaps, "is a 

 later development or, possibly, improvement upon gastrula- 

 tion.'") This gastrula stage is an open sac composed of two 

 layers of cells, the outer or ectoderm, and the inner or endoderm. 

 In most cases a third mass of cells, the mesoderm or middle 

 layer is formed, probably from the endoderm. The opening 

 of this sac-like body becomes the primitive mouth. 



History and Theories of Evolution. — Evolution is not a new 

 theory. Traces of such an idea are found in old Greek phil- 

 osophy. Empedocles (about 500 b. c.) believed that "plants first 

 sprang from the earth while the latter was in process of devel- 

 opment. After them came the animals, their parts having 

 first formed themselves independently and then been joined by 

 love." After ceaseless trials, nature succeeded in producing 

 fit tribes for perpetuation. Osborn says this is the germ of the 

 "survival of the fittest." Aristotle (384-322 b. c), though 

 believing in separate creation, taught vaguely that living baings 

 formed a gradual succession from the "less to the more perfect." 

 Aristotle laid great stress upon the inductive method of study, 

 and he was so great an observer of animals as well as a collector 

 of the statements of others concerning them that he has been 

 called the founder of zoology. If only his principle — that "we 

 must not accept a general principle from logic only, but must 

 prove its application to fact; for it is in facts that we must 

 1 Romanes' "Darwin and After Darwin." 



