EMBRYO OF TWENTY-SEVEN SEGMENTS 



Section through the Cranial Portion of the Open Intestine (Fig. 64). The intestine 

 is now open ventrad, its splanchnopleure passing directly over to that of the vascular area. 

 The folds of the amnion do not join, leaving the amniotic cavity open. The dorsal aorta 

 is divided by a septum into its primitive components, the right and left descending aortoe. 

 Lateral to the aortae are the small posterior cardinal veins. The ccelom is in communica- 

 tion with the extra-embryonic body cavity. 



Section through the Seventeenth Pair of Mesodermal Segments (Fig. 65). The 

 body of the embryo is now no longer flexed to the right. On the left side of the figure, the 



Mesodermal segment 



Nephrotome 

 Ccelom 



Spinal cord 



Ectoderm 



Somatoplenre 



'ephric duct 



'omatic mesoderm 



Splanchnopleure' 



Aorta and vitelline Notochord 

 artery 



FIG. 66. Transverse section of a fifty-hour chick embryo, at the level of the origin of the vitel- 



Hne arteries. X 50. 



mesodermal segment shows a dorso-lateral myotome plate. The median and ventral por- 

 tion of the segment is being converted into mesenchyme. On the right side appears a 

 section of the primary excretory, or mesonephric duct. The embryonic somalopleure is 

 arched and will form the future ventro-lateral body wall of the embryo. The lateral in- 

 foldings of the somatopleure give indication of the later approximation of the ventral 

 body walls, by which the embryo is separated from the underlying layers of the 

 blastoderm. 



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

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



Spinal cord 

 Descending aortc 

 Somatic mesoderm 



Ectoderm 



Segmental zone 



..Somatic mesoderm 



Splanchnic mesoderm 



Codom 



Notochord Entoderm 



FIG. 67. -Transverse section of a fifty-hour chick embryo through the segmental zone, caudal to 



the mesodermal segments. X 50. 



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

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

 side the connection 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- 

 Section Caudal to the Mesodermal Segments (Fig. 67). The mesodermal segments 

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

 derm from which later are formed the segments and nephrotomes. The notochord is larger, 



