868 A MANUAL OF PHYSIOLOGY 



axis ; its walls first come to overhang the furrow, and then to 

 coalesce ; and the medullary groove has now become the neural 

 canal. Immediately under it the mesoblast forms a rod of cells, the 

 notochord, which is the forerunner of the vertebral column ; around 

 this the bodies of the vertebrae are afterwards developed from cubical 

 masses of mesoblastic cells, arranged in pairs along the notochord, 

 and called the protovertebra. The rest of the mesoblast, running 

 out on each side from the protovertebrae, splits into two layers, an 

 upper or somatic layer, which unites with the epiblast, and a lower or 

 splanchnic layer ; which unites with the hypoblast. Between the two 

 layers is a space called the ccelom, or pleuro-peritoneal cavity 



(Fig. 335)- 



Up to the present, apart from the enclosure of the neural canal, 

 all this formative activity is buried beneath the surface of the blas- 

 toderm, and has not showed itself by any external token ; the 

 embryo still appears as a portion of the germinal area, and lies in its 

 plane. But now a pocket, or crease, or moat, beginning at the head 

 as the head-fold, then pushing under the tail, gradually creeps round 

 and undermines the whole embryo, which is raised above the general 

 level, and, as it were, scooped out from the rest of the blastoderm ; 

 till at length it lies on the latter, something like an upturned canoe, 

 enclosing a tube, complete in front and behind, but still open in the 

 middle, where it communicates with the interior of the yolk-vesicle. 

 Since this tube has been formed by the tucking in of the three 

 ancestral layers of the blastoderm, it follows that it is lined by hypo- 

 blast, supported externally by the splanchnic sheet of mesoblast. 

 So that now the body consists of a dorsal tube (the neural canal), 

 essentially of epiblastic origin, a ventral tube (the alimentary canal), 

 essentially of hypoblastic origin, and between the two a massive 

 double layer of mesoblastic tissue, which contributes supporting 

 elements to both. At this point it may be well to emphasize the 

 fact that this embryological distinction of the three primitive layers 

 has a deep and fundamental meaning, and corresponds to a physio- 

 logical distinction that endures throughout life. The hypoblast, the 

 lowest layer in position, may also be described as the lowest in the 

 physiological hierarchy. It furnishes the epithelial lining of the 

 alimentary canal from the beginniiag of the oesophagus to near the 

 end of the rectum, as well as the epithelium of the organs which 

 arise from diverticula of the primitive intestine, viz., the digestive 

 glands with the exception of the salivary glands, the lungs, and the 

 passages leading to them, the thyroid, and the greater part of the 

 thymus gland in its primitive condition before the lymphoid tissue 

 derived from the mesoblast has as yet grown into it. According 

 to some authorities, the notochord is also derived from the 

 hypoblast. 



Upon the whole, it may be said that the tissues of hypoblastic 

 origin are essentially concerned in chemical labours, in the absorption 

 of food material and excretion of waste products, The mesoblastic 

 tissues are essentially concerned in mechanical labour ; they are the 

 tissues of movement arid of passive support. The epiblastic tissues 

 are at the top of the pyramid ; they govern the rest. 



