NEURAL FOLDS 

 SOMATIC MESODERM 



SPLANCHNIC MESODERM 

 FOREGUT 



Fig. 125. A cross section through the level of the forebrain of a 25-hour 

 embryo reveals the heavily stained brain wall, the neural folds in apposition, and 

 the cavity of the brain. Surrounding the brain is loose mesenchyme. 



ECTODERM 

 SOMATIC MESODERM 



SPLANCHNIC MESODERM 



ENDODERM 

 AMNIOCARDIAC VESICLE 



SOMATOPLEURE 



SPLANCHNOPLEURE 



NOTOCHORD 

 EPIMYOCARDIUM 



Fig. 126. A section through the intestinal portal. The two large vesicles in 

 the mesoderm are the amniocardiac vesicles. Note the thick-walled epimyocar- 

 dium, which will form a part of the muscular layer of the heart. The epi- 

 myocardial layers move toward the midline and pinch together the endoderm, 

 thereby closing the gut cavity. See also Figure 104. 



Fig. 127. A section through a pair of somites ot a 25-hour chick embryo. 

 The neural folds are widely separated and the notochord is between the neural 

 tube and the endoderm. In some regions the separation of the mesoderm into 

 two layers can be seen. 



PRIMITIVE STREAK 



FlG. 128. The primitive streak in a 2 5 -hour embryo. The ectoderm, meso- 

 derm, and endoderm fuse into the streak. A primitive groove is visible. 



