EMHRYO OF TWKXTY-SEVEN SEGMENTS 65 



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

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

 The folds of the aiiiiiiini 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 aortas. 

 Lateral to the aorta? 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 Spinal cord 



Ectoderm 

 Nephrotvnif. \ / 



\ \ / iMesonephnc duct 



Cxlom ^ \ .^,itf^te4>^ / , Somatic mesoderm 



Somaloptnire 



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- 

 line 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 mcsoncphric 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 Ectoderm 



'ental 



Somatic mesoderm 



Descending aorta I / Segmental zone 

 Somatic mesoderm 



Splanchnic mesoderm / \ Clrlom 



Nolochord Entodi-rm 



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 segmctital zone, a somewhat triangular mass of undifferentiated meso- 

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

 s 



