CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 



73 



:tion 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 nephrotomes. The notochord is larger, 



Ales. Segment" 

 Descending aorta 



Somcdopleure 

 Som. mes. 



Sp'mal cord 



Ectoderm 

 Note chord 



Somatic mesoderm 



Splanchnic mesoderm 



^Entoderm 



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



embryo. X 50. 



Sp'ina I cord 



Ectoderm 



Nephrotome 



Somatic mesoderm 



Somatopleure. 



tSplanchnopleure 



Aorla JJ- Vitelline artery 



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



arteries. X 50. 



Descending aorta 

 Sim.t7iesod.erm 



Splanchnic, mesoderm ., / \r . 



Notochord J \Entoderm 



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



X 50. 



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

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



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, 



