ONTOGENY OF METAZOA 155 



(co) is formed between the body wall and that of the 

 digestive tube. Organs which develop later as outgrowths 

 from the alimentary canal must push aside the membrane 

 lining the coelom, and on this account the membrane en- 

 velops all the internal organs closely. For example, the 

 peritoneum, or lining of the ccelom, surrounds all the ab- 

 dominal organs in a man's body. This makes it difficult 

 to perform surgical operations, for, if bacteria gain access 

 to the ccelom, they will cause peritonitis (inflammation of 

 the peritoneum) and the infection will wander about among 

 the internal organs, usually causing death. 



6. The Formation of Systems of Organs. The germ layers 

 play rather definite roles in subsequent development. The 

 nervous tissues and the outer integument, or skin, always 

 arise from the ectoderm. The entoderm forms the lin- 

 ing for the middle portion of the alimentary canal, or midgut. 

 Other tissues and systems of organs take their origin 

 chiefly from the mesoderm. The greater part of the body 

 is therefore formed, in most animals, from the middle germ 

 layer. 



This brief synopsis indicates the chief steps in metazoan 

 development : Maturation, fertilization, cleavage, gastrula- 

 tion, and ccelom formation. The first three stages occur 

 in protozoans as well as metazoans, but it is only in many- 

 celled animals that tissue differentiation takes place after 

 the formation of the primary germ layers by gastrulation. 

 As has been stated, some metazoans never get much beyond 

 the gastrula stage, but even those that pass on to more 

 specialized conditions (ccelom, paired appendages, chorda, 

 etc.) follow the regular route through the stages of matura- 

 tion, fertilization, cleavage, and gastrulation. This per- 

 sistent adherence to one conventional type of develop- 

 ment is believed to indicate genetic relationship among 

 animals. According to the Law of Biogenesis, therefore, 

 metazoans arose from protozoans; all were at one time (and 

 some still are) gastrulas, and many have through speciali- 

 zation gone farther. The racial development of all the 



