14 MORPHOGENESIS 



whose edges (neural ridges) by local growth are folded upward so as to meet in 

 the mid-line where they fuse, thus transforming the groove into the neural tube 

 (figs. 11, 12, 13, 15, 16, 17, 18). 



The closure begins, not at the anterior end (as might be expected from the general law of 

 cranio-caudal development), but in the cervical region, extending forward into the brain region, 

 and backward along the spinal cord. Thus the extreme ends (anterior and posterior neuropores) 

 are the last to close. 



The precocious and energetic growth of the neural anlage is largely responsible for the ven- 

 tral flexure of the embrj^onic body axis, especially in the head region, where the flexures of the 

 brain are very conspicuous (figs. 22, 26). 



With the closure of the neural tube dorsally and of the aKmentary canal ventrally the human 

 embryo assumes the typical vertebrate form. The cylindrical body wall now encloses two tubes 

 (neural and enteric) with the longitudinal axis (notochord) between them (figs. 18, 24). 



After the embryonic disc has been transformed into a tube, the body of the human embryo 

 in cross section appears not circular but elongated dorso-ventrally. This is the typical form for 

 vertebrates with horizontal body axis. In later foetal stages, the body becomes more rounded in 

 cross section, and finally, with the assumption of the erect posture in postnatal life, becomes 

 decidedly flattened_dorso-ventrally (figs. 20, 21). 



Fig. 17. — A Hdman^Embryo 2.5 mm. in Length. (After Kollmann.) 



Heart 



Mesodermic somite . "" ' Umbilical vein 



Medullary canal f — ,. 



Development of the mesoderm. — The mesodermic layer on each side of the 

 notochord in the embryonic disc develops in two divisions. The medial (or 

 dorsal) divisions form a series of hollow segments, the somites (figs. 16, 17, 18). 

 Tlio lateral (later ventral) divisions each split into an upper (outer) or somatic layer 

 and a lower (inner) or visceral layer. When the embryonic disc becomes folded, 

 the corresponding somatic and visceral layers unite ventrally and enclose between 

 them the common ccclom or primitive body cavity (fig. 18). 



As previously noted, the mesoderm arises chiefly from the lateral portions of the 'head 

 process.' A comparatively early stage before the appearance of the somites is shown in cross 

 section in fig. 15. The somites appear first in the occipital region, and rapidly differentiate 

 succcHHively in the cranio-caudal direction (figs. 16, 17, 22). In embryos 7 or 8 mm. in length, 

 about 40 somites may be distinguished, '.i to 5 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral 

 and 5 or 6 coccygeal (in the rudimentary tail region). • 



The cceUrm or body cavity is un.segrnented. Two primitive pericardial cavities appear sepa- 



