DEVELOPMENT OF THE LEOPARD FROG 



417 



differentiate in place within the mesoderm, and hkewise the 

 skeleton and muscles. 



If one considers the histological structure of the adult, the 

 epidermis that covers the entire body arises from the ectoderm of 

 the embryo, the mucous membrane of the digestive tract from the 

 endoderm. Everything between these two thin layers of cells 

 is mesodermal in origin, with a few exceptions such as the cells 

 lining the glands of the skin (cf. Fig. 52, p. 95) or the lungs and 

 other organs formed by evagination. The dermis of the skin, the 

 muscles and the connective tissue of the body wall, and the parietal 

 peritoneum originate from the outer layer of the mesoderm in the 

 embryo {cf. Fig. 216 C); the visceral peritoneum, muscle layers 

 and submucosa of the digestive tract, from the inner layer of the 

 mesoderm. 



Thus there are differentiated, at the time of gastrulation, three 

 layers of cells, the ectoderm, endoderm, and mesoderm. These 

 were called germ layers by the embryologists of the early nine- 

 teenth century because of their la3^er-like arrangement in the 

 embryo. From these germ-layers the organs and tissues of the 

 body originate in the manner described, as summarized in the 

 accompanying table (Fig. 218). 



Origin of Adult Structure.s from Germ Layers i.v Frog 



Fig. 218. — Germ layers of frog and their derivatives (cf. F^g. 149 p, 309). 



