EMBRYO OF SEVENTEEN SEGMENTS 51 



gut (cf. Fig. 42). Two sections caudad is found the opening (intestinal portal) where the 

 fore-gut communicates with the flattened open gut between the entoderm and the yolk. 

 On each side of the fore-gut are the large mtelline veins, sectioned obliquely. The 

 splanchnic mesoderm overlying these veins is pressed by them against the somatic 

 mesoderm and the cavity of the ccelom is thus interrupted on each side. 



Neural tube 



Mesodermal segment 



Neural cavity 



Ectoderm 

 Notochord 



Dorsal aorta 



matopleure 



Coslom 

 Splanchnic mesoderm 



R. vitelline vein 



Splanchnopleure Open gut Entoderm 



FIG. 48. Transverse section caudal to the intestinal portal of a thirty-eight-hour chick embryo. 



X 90. 



Section Caudal to the Intestinal Portal (Fig. 48). This section resembles the preceding 

 save that the primitive gut is without a ventral wall. The right mtelline vein is still large. 



Section through the Fourteenth Pair of Primitive Segments (Fig. 49) . The body of 

 the embryo is now flattened on the surface of the yolk. Here the descending aortae are 



Neural tube 



Mesodermal segment 

 Central cells of segment 



Somatic mesod< 



S-blanchnic mesoderm 



Descending aorta 



Ectoderm 



Pronephric tubule 



Entoderm 



Notochord 



FIG. 49. Transverse section through the fourteenth pair of mesodermal segments of a thirty- 

 eight-hour chick embryo. X 90. 



still separate and occupy the depressions lateral to the primitive segments. The section 

 is characterized by the notochord and the differentiated mesoderm which forms the primi- 

 tive segments, nephrotomes, and somatic and splanchnic mesoderm. Arising from the 

 nephrotomes are sprout-like pronephric tubules. The tips of these hollow out and unite to 

 form the primary excretory, or mesonephric duct. All of these structures are described on 

 PP- 53-54- 



