24 STRUCTURE OF THE VERTEBRATES 



neural tube which becomes the nerve cord of the animal. 



The notochord begins its development slightly later than the 

 nerve cord. It is formed from the dorsal cells of the archenteron. 

 These endodermal cells develop a groove, the curve being exactly 

 opposite to that of the nerve cord, and the lateral margins grow 

 downward until they meet. The notochord becomes a continuous 

 loose rod of cells beneath the neural groove. 



The mesoderm arises from the primitive endoderm as paired, 

 lateral pouches from the gut. It should be kept in mind that 

 though the nerve cord and notochord are continuous structures, 

 the mesodermal pouches are a series of discontinuous outgrowths, 

 or enterocoels (literally ^'gut pouches"). Thus metamerism, or 

 segmentation, has its origin in the mesoderm. Only three pairs 

 of pouches arise directly from the archenteron, the others de- 

 veloping by the growth and division of the third (most posterior) 

 primitive pouch. 



Soon the lining of the gut fuses along the dorsal border cut- 

 ting the notochord off as a separate structure ; and following this 

 the mesodermal somites lose their connection with the enteron. 

 The embryo is now an ellipsoid organism completely enclosed by 

 ectoderm, for as the neural folds closed along the dorsal mid- 

 line they also overgrew the blastopore. Therefore, for a short 

 time, the neural canal is continuous with the enteric canal. 

 Later the posterior end of the nerve cord closes, and the blasto- 

 pore breaks through the ectoderm to form the anus, or posterior 

 opening of the enteron. 



A coelomic cavity develops through the enlargement of the 

 original enterocoel cavities. As the metameres grow downward 

 the ventral portion becomes thin-walled, and the dorsal part of 

 each thickens into a mass of tissue. The dorsal segment is the 

 epimere which gives rise to the muscle tissues and the connec- 

 tive tissues; the ventral hypomere forms the smooth muscle of 

 the digestive tract, the heart, the mesenteries, and the lining of 

 the coelomic cavity. These relationships can be studied in the 

 illustrations. Observe that as the hypomere pushes ventrally 

 the median layer of cells is pushed against the gut, which has 

 separated from the notochord. During this process the dorsal 

 epimere has grown around the notochord and also ventrally, 

 forcing its way between the ectoderm and the lateral layer of 



