central mass 



trophoderm 

 blastodisc 



disintegrating trophoderm 



entoderm cells 



amniotic cavity (ectoderm lined) 

 syntrophoblost^ 



blastodisc 



caudal thickening 

 trophoblast 



trophoderm 



mesoderm 



amniotic fold at head end 



Hensen's node 



primitive streak coelom 



extraembryonic coelom 



entoderm 



primitive groove 



amniotic cavity 



coelom 



Hensen's node 

 neural plate \ P'' 



Figure 7-15. Stages of mammols. A, early blastula of pig with central mass; B, central mass cells 

 forming blastodisc and entoderm; C, fully formed blastodisc without overlying trophoderm; D, 

 sagittal section of blastodisc beginning gostrulation; E, early gastrulation; F, primitive streak stage 

 and formation of head and toil fold of amnion; G, cross section of primitive streak of pig; 

 H, formation of amniotic and yolk-sac vesicles and extraembryonic mesoderm from central mass in 

 human; I, blostodisc development in human and differentiation of syntrophoblost and trophoderm; 

 J, primitive streak stage of human (gastrulation of embryonic mesoderm and notochord). 

 (After Patten, 1946) 



210 • EMBRYOGENESIS OF THE CHORDATES 



