REPRODUCTION. 



661 



from their internal surfaces. These layers are known from without 

 inward as the epiblast, mesoblast, and hypoblast. 



The epiblast gives rise to the central nerve system, the epidermis 

 and its appendages, and the primitive kidneys. 



The mesoblast gives rise to the dermis, muscles, bones, nerves, 

 blood-vessels, sympathetic nerve system, connective tissue, the 

 urinary and reproductive apparatus, and the walls of the alimentary 

 canal. 



The hypoblast gives rise to the epithelial lining of the alimentary 

 canal and its glandular appendages, 

 the liver and pancreas, and the epithe- 

 lium of the respiratory tract. 



Dorsal Laminae. As develop- 

 ment advances, the true medullary 

 groove deepens, and there arise two 

 longitudinal elevations of the epiblast 

 the dorsal lamina, one on either side 

 of the groove, which grow up, arch 

 over, and unite so as to form a closed 

 tube, the primitive central nerve 

 system. 



The Chorda Dorsalis. Just be- 

 neath the neural canal there arises a 

 group of mesoblastic cells which ar- 

 range themselves in the form of a 

 cylindric rod, which marks out the 

 position of the future bony axis of the 

 body. This rod is known as the 

 chorda dorsalis or notochord. 



Primitive Vertebrae. On either 

 side of the neural canal the cells of 

 the mesoblast undergo a longitudinal 

 thickening, which develops and extends 

 around the neural canal and the 

 chorda dorsalis, and forms the arches * 



and bodies of the vertebrae. They become divided transversely 

 into segments. 



The mesoblast now separates into two layers : the external, joining 

 with the epiblast, forms the somatopleura; the internal, joining with 

 the hypoblast, forms the splanchno pleura; the space between them 

 constitutes the pleuro- peritoneal cavity (Fig. 327). 



Visceral Laminae. The walls of the pleuro-peritoneal cavity 

 are formed by a downward prolongation of the somatopleura (the 

 visceral lamina), which, as they extend around in front, pinch off 

 a portion of the yolk-sac (formed by the splanchnopleura), which 



FIG. 327. DIAGRAM REPRESENT- 

 ING THE RELATION OF PRIM- 

 ARY STRUCTURES IN A DEVEL- 

 OPING CHICKEN; VERTICAL 

 TRANSVERSE SECTION. The 

 medullary groove and chorda 

 dorsalis are seen in section; 

 the alimentary canal pinched 

 off from the yolk-sac is com- 

 pletely closed, a. Amnion. 

 a, c. Amniotic cavity filled 

 with amniotic fluid, pp. 

 Space between amnion and 

 chorion continuous with the 

 pleuro-peritoneal cavity in- 

 side the body. vt. Vitelline 

 membrane, or zona pellucida. 

 ys. Yolk-sac, or umbilical 

 vesicle. (Foster and Balfour.) 



