THE VISCERAL ARCHES AND FURROWS. 



59 



From the twenty-third to the twenty-eighth day the excessive flexion gradually 

 disappears, owing to the increased volume of the heart and the growth of the head, 

 and by the end of the fourth week the embryo has acquired the most characteristic 

 development of the embryonic stage (Fig. 71). The reduction in the curvature 

 of the body-axis and the consequent separation of its poles and the raising of the head 

 are accompanied by the appearance of four well-marked axial flexions, the cephalic, 

 the cervical, the dorsal, and the ^sacral flexures (Fig. 71). The first of these, the 

 cephalic, is an accentuation of the primary flexure, which is seen as early as the 

 eighteenth day, and is indicated by the projection of the midbrain ; it corresponds 

 in position to the future sella turcica. The second and very conspicuous bend, the 

 cervical flexure, marks the caudal limit of the cephalic portion of the neural axis, 

 and agrees in position with the subsequent upper cervical region. The dorsal and 

 sacral flexures are less well defined, the former being situated opposite the upper 

 limb-bud, where the cervical and dorsal series of somites join, the latter, near the 

 lower limb-bud, corresponding with the junction of the lumbar and sacral somites. 



The cephalic segment at this stage presents numerous prominent details, the 



Fig. 71- 



Cervical flexure 



6tic vesicle- 



Maxillary process of first- 

 visceral arch 



Third visceral arch 



First external visceral furrow 



Second visceral arch 



Mandibular process of first 

 •4 « visceral arch 



Dorsal flexure 



Upper limb 



Heart 



Lower limb 



Sacral flexure 

 Human embryo of about twenty-eight days, drawn from the model of His. X lo. 



secondary cerebral vesicles, the forebrain, the interbrain, the midbrain, the hind- 

 brain, and the after-brain, the visceral arches and furrows, the optic and otic vesicles, 

 the olfactory pits, and the primitive oral cavity all being conspicuous. The heart ap- 

 pears as a large protrusion, occupying the upper half of the ventral body-wall, on 

 which the primary auricular and ventricular divisions are distinguishable. The somites 

 form a conspicuous longitudinal series of paraxial quadrate areas, about thirty-seven 

 in number ; they correspond to the intervertebral muscles, and may be grouped to 

 accord with the primary spinal nerves, being, therefore, distinguished as eight cer- 

 vical, twelve dorsal, five lumbar, five sacral, and five or more coccygeal somites. 



THE VISCERAL ARCHES AND FURROWS. 



Since the visceral arches are best developed in the human embryo during the 

 last half of the third week, a brief consideration of these structures in this place is 

 appropriate. The visceral arches in mammalian embryos constitute a series of five 

 parallel bars separated by intervening furrows, obliquely placed on the ventro-lateral 

 aspect of the cephalic segment, occupying the region which later becomes the 

 neck. They represent, in rudimentary development, the important branchial or gill- 



