58 



HUMAN ANATOMY. 



Fig. 69. 



Otic vesicle 



Cephalic flexure 



Optic vesicle 



Maxillary process 



Mandibular proces: 

 of first visceral arch 

 Caudal end ol 

 embryo 

 Umbilical cord in- 

 section 



Lower limb-bud 



__4_U-Heart 



Second visceral arch 



Third visceral 

 arch 



Fourth visceral 

 arch 



Upper limb-bud 



Human embryo of about twenty-three days, drawn from the model 

 of His. X 10. 



The more complete differentiation of the digestiv^e tube and the ventral folding in of 

 the body-walls change this relation, the rapidly decreasing umbilical vesicle soon 

 becoming secondary to the embryo. 



At the close of the stage of the blastodermic vesicle — about the fifteenth day — 

 the embryo possesses a general cylindrical body-form, the dilated cephalic pole being 

 free, while the belly-stalk attaches the caudal segment to the chorion ; the amniotic 

 sac invests the dorsal aspect, the large umbilical vesicle occupying the greater part 



of the ventral surface. Human 

 embryos of the fourteenth and 

 fifteenth days (Fig. 68, 3 and 4) 

 are distinguished by a conspicu- 

 ous flexure opposite the attach- 

 ment of the umbilical vesicle, the 

 convexity being directed ven- 

 trally, the deep corresponding 

 concavity producing a marked 

 change of profile in the dorsal 

 outline. During these changes 

 the expansion of the cerebral 

 segments outlines the three pri- 

 mary divisions of the cephalic 

 portion of the neural tube, the 

 ayiterior, the middle, and the 

 posterior brain-vesicles. 

 A little later a series of conspicuous bars, the visceral arches, appears as ob- 

 liquely directed parallel ridges on either side of the head, immediately above the 

 prominent heart-tube, which is now undergoing marked torsion. By the nineteenth 

 day the dorsal concavity, which is peculiar to the human embryo, has entirely disap- 

 peared, the profile of this part of the embryo presenting a gentle convexity ; the 

 cephaHc axis, however, exhibits a marked bend, the cephalic flexiire, in the vicinity 

 of the middle cerebral vesi- 

 cle, in consequence of which 

 the axis of the anterior cere- 

 bral segment lies almost at 

 right angles to that of the 

 middle vesicle. The com- 

 pletion of the third week 

 finds the characteristic de- 

 tails of the cephalic end of 

 the embryo, the cerebral, 

 the optic, and the otic vesi- 

 cles, and the visceral arches 

 and intervening furrows well 

 advanced, with correspond- 

 ing definition of the primitive 

 heart and the umbilical stalk 

 and vesicle. The limb-buds 

 usually appear about this 

 time, those of the upper ex- 

 tremity slightly preceding 

 those of the lower. 



The period between the 

 twenty-first and the twenty-third days witnesses remarkable changes in the general 

 appearance of the embryo ; in addition to greater prominence of the visceral arches, 

 the cerebral segments, and the limb-buds, the embryonic axis, which, with the 

 exceptions already noted, up to this time is only slightly curved, now undergoes 

 flexion to such extent that by the twenty-third day the overlapping cephalic and 

 caudal ends of the embryo are in close apposition, the body-axis describing rather 

 more than a complete circle (Fig. 69). • 



Fig. 70. 



Otic vesicle 



Cephalic flexure 



Optic vesicle 



Mandibular process 



of first visceral arch 



Olfactory pit 



Umbilical cord 



Cervical flexure 

 / >Second visceral arch 



/°^C/Third visceral arch 

 ^'^^^^Founh visceral arch 



^ Heart 



Upper limb-bud 



Lower limb-bud 



Human embryo of about twenty-five davs, drawn from the model of His. 



X 10. 



