64 



THE STUDY OF CHICK EMBRYOS 



Section Caudal to the Mesodermal Segments (Fig. 67).— 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 nephroiomes. The notochord is 



Mesodermal segment Spinal cord 

 ,^ , V I Ectoderm 



Nephrotomet \ j / ,, , ■ j , 



' ' ' ' tMesonephnc duel 



„, _„ . y Somatic mesoderm 



Somatopleure 



Splanchnopleure' 



Entoderm 



Aorta and vitelline Notochord 

 artery 



Fig. 66. — Transverse section of a fifty-hour chick embryo at the level of the origin of the vitelUne arteries. 



X50. 



larger, the aortse smaller, and a few sections caudad they disappear. Laterally the somato- 

 pleure and splanchnopleure are straight and separated by the slit-like coelom. 



Section through the Primitive Knot Cranial to the Hind-gut (Fig. 68). — With 

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



Spinal cord 

 Descending aorta 

 Sofnatic mesoderm 



Ectoderm 



zone 

 •Somatic mesoderm 



Splanchnic mesoderm 



Ccelom 



Notochord 



Entoderm 



Fig. 67. — Transverse section of a fifty-hour chick embryo through the segmental zone caudal to the meso- 

 dermal segments. X SO. 



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

 but its ventral portion, the notochord, mesoderm, and entoderm, blend in a dense mass of tissue 

 which is characteristic of the primitive knot. Laterally the segmental zone and the various 

 layers are differentiated. 



Neural tube Ecloaerm 



Segmental zone 

 Coilom — 



Splanchnopleure 



Entoderm 



Notochordal plate 



Fig. 68. — Transverse section of a fifty-hour chick embryo through the primitive node cranial to the 



hind-gut. X 50. 



Section Passing through the Hind-gut (Fig. 69).— In this embryo the caudal evagina- 

 tion to form the hind-gut has just begun. The section shows the small cavity of the hind- 



