HUMAN EMBRYOLOGY. 



Mesoderm of. 



ainnion 

 Ectoderm of 



amnion 



Neural crest 



Eoof-plate 



Lateral wall of 

 neural groove 



Floor-plate 



Mesoderm of 

 entoderm vesicle 



Entoderm 



Cavity of 

 entoderm vesicle 



vity 



FIG. 35. TRANSVERSE SECTION OF A ZTGOTE, showing early stage 

 of embryonic secondary mesoderm before the appearance of the 

 embryonic parts of ccelom. 



^Amnion cavity 

 Paraxial 

 erm 



Embryonic 

 ccelom 



Trophoblast of ' 



chorion 



Mesoderm of 



chorion 



Mesoderm of 

 amnion - 



Ectoderm of 

 amnion - 



Neural crest^ 



Roof- plate 

 Lateral wall of^ 

 neural groove^. 



Floor-plate 

 Entoderm 



Mesoderm of 

 ntoderm vesicle^ 

 Cavity of 

 ntoderm vesicle 



FIG. 36.- TRANSVERSE SECTION OF A ZYGOTE, showing early 

 stage of development of embryonic coelom and differentiation 

 of mesoderm. 



Neural crest* 



Trophoblast of chorion 

 Paraxial mesoderm 



Notoeho: 



QSO. of yolk-sac 

 ntoderm of yolk-sac 



Intermediate cell tract 

 Splanchnic and 

 somatic layers of 

 lateral plate meso. 



Embryonic 

 coelom 



> Amnion 

 cavity 



Extra-embryonic 

 coelom 



Alimentary canal 

 Yolk-sac 



FIG. 37. TRANSVERSE SECTION OF A ZYGOTE, showing union of 

 intra- and extra-embryonic parts of coelom and separation of 

 embryonic mesoderm into paraxial bars, intermediate tracts, 

 and lateral plates, with separation of lateral plates into 

 somatic and splanchnic layers by the intra-embryonic part of 

 the coelom. 



appears ; this is the primitive 

 streak(Figs.31, 34). It is formed 

 by the proliferation of the ecto- 

 mesodermal cells of the wall of 

 the larger inner vesicle. The 

 deeper cells of the streak, which 

 displace the primary mesoderm 

 from the median plane, and thus 

 come into contact with the 

 entoderm, are the rudiments 

 of the secondary or primitive 

 streak mesoderm (Fig. 34). 

 The superficial cells form part 

 of the surface ectoderm of the 

 embryo. 



At the anterior end of the 

 primitive streak the mesoder- 

 mal elements of the streak fuse 

 with the subjacent entoderm 

 and through the fused mass 

 a perforation, the neurenteric 

 canal (p. 23), is formed (Fig. 

 32). 



The canal itself soon dis- 

 appears, but the cells of its 

 walls form a nodal growing 

 point, and by their proliferation 

 the length and breadth of the 

 embryonic area are increased. 

 The mesoderm cells proliferated 

 from the cephalic border of the 

 nodal point are the rudiments 

 of the notochord, which has 

 already been considered (p. 

 24). 



It is uncertain whether or 

 not the mesodermal cells 

 budded off from the nodal point 

 blend with the cells of the 

 primary mesoderm, but there 

 can be little doubt that they 

 form by far the greater part, 

 if not the whole, of the perma- 

 nent mesoderm of the embryo. 



Either by displacement or 

 by union with the primary 

 mesoderm the secondary meso- 

 derm forms a continuous sheet 

 of cells, in the embryonic area, on 

 each side of the median plane. 



Each of the lateral sheets 

 is thickest where it abuts 

 against the notochord and the 

 wall of the neural groove, and 

 thinnest at its peripheral 

 margin, where it is continuous 

 with the primary mesoderm of 

 the extra-embryonic area (Fig. 

 35). 



