EMBRYO' OF TWENTY-SEVEN SEGMENTS 



63 



the future ventro-lateral body wall of the embryo. The fold lateral to the arch of the somato- 

 pleure gives indication of the later approximation of the ventral body walls, by which the 

 embryo is separated from the underlying layers of the blastoderm. 



Splanchnic mesoderm — -^ 



Ectoderm 



Posterior cardinal vein- 

 Notoehord- 



Somatoplcure' 



Open gill 



Entoderm 



Cliorion 



Amnion 

 Central canal 

 '^Spinal cord 



Mesodermal segment 

 R. descending aorta 



Ca'lom 

 Somalopleure 



Fig. 64. — Transverse section through the cranial portion of the open intestine of a fifty-hour chick 



embryo. X 50. 



Section through the Origin of the Vitelline Arteries (Fig. 66). — At this level the 

 embryo is more flattened and simpler in structure, the section resembling one through the 

 mid-gut region of a thirty-eight-hour chick (Fig. 49). The amniotic folds have not appeared. 



Mesodermal segment 

 Descending aorta- 

 Somatopleure 

 Somatic mesoderm 



Spinal cord 



Ectoderm 

 Notoehord 



Somatic mesoderm 



Splanchnopleure'^ / \ ^Splanchnic mesoderm 



Cmlom Entoderm 



Fig. 55. — Transverse section through the seventeenth pair of mesodermal segments of a fifty-hour chick 



embryo. X 50. 



On the left side of the figure the vitelline artery leaves the aorta. On the right side the con- 

 nection of the vitelline artery with the aorta does not show, as the section is cut somewhat 

 obliquely. The posterior cardinal vein is present just laterad of the right mesonephric duct. 

 The other structures were described in connection with Fig. 49. 



