REPRODUCTION 1009 



area and into the rest of the blastodermic vesicle, the ectoderm in 

 front of the primitive streak rises up in two lateral ridges, enclosing 

 between them the medullary groove. The medullary groove is the 

 beginning of the cerebro-spinal 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 mesoderm 

 forms a rod of cells, the notochord, which is the forerunner of the 

 vertebral column ; around this the bodies of the vertebrae are after- 

 wards developed from cubical masses of mesodermic cells, arranged 

 in pairs along the notochord, and called the protovertebrce. The rest 

 of the mesoderm, running out on each side from the protovertebrae, 

 splits into two layers, an upper or somatic layer, which unites with 

 the ectoderm, and a lower or splanchnic layer, which unites with the 

 endoderm. Between the two layers is a space called the coelom, or 

 pleuro-peritoneal cavity (Fig. 445) . 



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

 all this formative activity is buried beneath the surface of the 

 blastoderm, 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 endoderm, supported externally by the splanchnic sheet 

 of mesoderm. So that now the body consists of a dorsal tube (the 

 neural canal), essentially of ectodermic origin, a ventral tube (the 

 alimentary canal), essentially of endodermic origin, and between 

 the two a massive double layer of mesodermic tissue, which con-, 

 tributes 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 physiological distinction that endures throughout 

 life. The endoderm, 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 beginning 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 mesoderm has as yet grown 

 into it. According to some authorities, the notochord is also 

 derived from the endoderm. 



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

 origin are essentially concerned in chemical labours, in the absorp- 

 tion of food material and excretion of waste products. The meso- 

 dermic tissues are essentially concerned in mechanical labour ; they 

 are the tissues of movement and of passive support. The ectodermic 

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



From the mesoderm arise the muscles, the entire vascular system, 

 with its blood- and lymph-corpuscles, the bones and connective 



