CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 



73 



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

 obliquely. The other structures were described in connection with Fig. 47. 



Section Caudal to the Mesodermal Segments (Fig. 64).— The mesodermal seg- 

 ments are replaced by the segmental zone, a somewhat triangular mass of undifferentiated 

 mesoderm from which later are formed the segments and nepkrotomes. The notochord is larger, 



Mes. Segment" 

 Descending aorta 



Somcdopleure 



Son. mes 



Spinal cord 



Ectoderm 

 Nolo chord 



Somatic mesoderm 



Splanchnopleure 

 Coelo 



^Splanchnic mesoderm 



^Entoderm 



Fig. 62. — Transverse section through the seventeenth pair of mesodermal segments of a fifty-hour chick 



embryo. X 50. 



Mes.segment 

 Nephrott 



Spina I cord 



Ectoderm 



Somatopkurfi 



Nephrotome 



Somatic mesoderm 



Splanchnopleure 



Aorta % Vitelline artery 



Fig. 63.— Transverse section of a fifty-hour chick embryo at the level of the origin of the vitelline 



arteries. X So. 



Descending aorta 

 Sim.tmsoderm 



Spinal cord 



Ectoderm 



Mes. Segment 



Somatic mesoderm 



Splanchnic- mesoderm 



Coelom 

 Entoderm 



Notochord 



Fig. 64. — Transverse section of a fifty-hour chick embryo through the last pair of mesodermal segments. 



X50. 



the aortas smaller, and a few sections caudad they disappear. Laterally the somatopleure and 

 splanchnopleure are straight and separated by the slit-like ccelom. 



Section through the Primitive Node Cranial to the Hind-gut (Fig. 65). — With 

 the exception of the ectoderm, the structures near the median line are merged into an undiffer- 

 entiated mass of tissue. The cavity of the neural tube and its dorsal outline may still be seen, 



