7 o6 A MANUAL OF VETERINARY PHYSIOLOGY 



vesicle ; this disc is the germinal area. In this area the embryo 

 develops. A development of the cells of the epiblast leads to 

 the formation of a groove in the area, known as the primitive 

 groove ; the sides of the groove grow up, meet, and enclose a 

 space, the neural arch, and from this arises the cerebro-spinal 

 system. A third layer of cells now develops between the epiblast 

 and hypoblast, and is spoken of as the mesoblast. 



The mesoblast furnishes a layer of cells under the neural canal, 

 the notochord (Fig. 246) ; from these the bodies of the vertebrae 

 develop. The mesoblast splits : the outer layer of it unites 

 with the epiblast, and so forms the somatopleure ; the inner 

 unites with the hypoblast, and forms the splanchnopleure. 

 As the result of the splitting, a cavity is formed between the 

 layers, known as the pleuro-peritoneal space, or ccelom (Fig. 246). 

 This is the body cavity of the future embryo. From the three 

 layers of cells developed around the ovum the following tissues 

 of the embryo are formed : 



From the Epiblast : 



The whole of the nervous system, both cerebro-spinal and 



sympathetic. 

 The epithelial sensory end-organs of the nerves of special 



sense, and the crystalline lens. 

 The epidermis and its appendages — hair, hoof, claws, horns. 

 The epithelium of all glands opening on the surface of the 



skin, including mammary, sweat, and sebaceous. 

 The epithelium of the mouth (not the tongue) and glands 



opening into it, the enamel of the teeth, and the epithelium 



of the anus. 

 The epithelium of the nasal passages and facial sinuses. 



From the Mesoblast : 



The bones, cartilages, and connective tissues of the entire 



body. 

 The whole of the muscles, skeletal and visceral. 

 The entire vascular and lymphatic system — blood-corpuscles, 



spleen, and serous membranes. 

 The generative organs and generative elements. 

 The urinary organs. 



From the Hypoblast : 



The epithelium of the alimentary canal and the ducts of all 



glands opening into it. 

 The epithelium of the respiratory organs. 

 The cells of the liver and pancreas. 

 The epithelium of the bladder, ureters, thyroid body, and 



part of thymus gland. 



During the growth of the mesoblast the embryo, which is 

 developing in the germinal area, is gradually being lifted off the 

 blastodermic vesicle by the formation of a sulcus or depression 

 which extends around the embryo. The embryo at this period 



