The Animal Organism and its Germ-Layers 47 



many groups split into two, thus making a four-layered 

 stage) ; and the innermost, called the endoderm. The deriva- 

 tives of these layers, as typically stated, are: From tlie 

 ectoderm, "The epidermis and its ajjpendages, hairs, nails, 

 epidermal glands, and the enamel of the teeth. The mucous 

 membrane lining the mouth and the nasal cavities, as well 

 as that lining the lower part of the rectum. The nervous 

 system and the nervous elements of the sense-organs, to- 

 gether with the lens of the eye." From the endodenn: "The 

 mucous membrane lining the digestive tract in general, to- 

 gether with the epithelium of the various glands associated 

 with it, such as the liver and pancreas. The lining epithe- 

 lium of the larynx, trachea, and lungs. The epithelium of 

 the bladder and urethra." From the mesodenn: "The vari- 

 ous connective tissues, including bone and the teeth (except 

 the enamel). The muscles, both striated and non-striated. 

 The circulatory system, including the blood itself and the 

 lymphatic system. The lining membrane of the serous cavi- 

 ties of the body. The kidneys and ureters. The organs of 

 reproduction." ^ 



The summary here given is taken from The Development 

 of the Human Body, by J. Playfair McMurrich, and conse- 

 quently has special application to man ; but it is a presen- 

 tation of what is usually understood to be contained in the 

 germ-layer theory applicable to all the metazoa with cer- 

 tain general modifications for the groups like the coelcnter- 

 ata which never advance to the three-layered condition. As 

 thus treated the germ-layers are structures as indubitably 

 as are bones or muscles or feet or hands or brains ; and the 

 now unquestioned fact that they are so alike in both struc- 

 ture and relations in so great a range of animals, and give 

 rise with such constancy to the corresponding parts of 

 the completed animals, has been and ever must be of great 

 importance for the interpretation and comprehension of the 

 vast complexity of animal structure. Says one of the fore- 



