CHICK EMBRYO OF EIGHTEEN PRIMITIVE SEGMENTS 



6l 



Transverse Section through the Primitive (Hensen's) Node or Knot (Fig. 

 49). The section shows the three germ layers bound together at the "knot" or node into a 

 mass of undifferentiated tissue. The mesoderm is split laterally into the somatic and 

 splanchnic layers. 



Transverse Section through the Primitive Streak (Fig. 50). In the mid-dorsal 

 line is the primitive groove. The germ layers may be seen taking their origin from the undiffer- 



Ecfod, 



Neural tube 



Neural Crest 



Sey menial zone 



Somaf/c mesoderm 



.".-*-.-- ,,:, ' - * ijr<iB*vi.j^_ 



Splanchnic mesoderm I M+ L . \r+j ^ Blood vessel 



Loelom' /Vorocfjora 1 \EnToderm 



FIG. 48. Transverse section through the rhomboidal sinus of a thirty-six-hour chick embryo. X 90. 



Ectoderm 



Somatic /n.ft$< 



Primitive node 



Coelom 



i 



Entoderm / Splanchnic, mesoderm 



FIG. 49. Transverse section through the primitive (Hensen's) node of a thirty-six-hour chick embryo. 



X 90. 



Somatic, mesoderm 



Primitive groove 



Coelom 

 SrJanchnopleure / \ _ . . - , . 



Entoderm / Primitive 3treaK Oplancnnic mes. 



IG. 50. Transverse section through the primitive streak of a thirty-six-hour chick embryo. X 90. 



entiated tissue of the primitive streak beneath the primitive groove. Between the splanchnic 

 mesoderm and entoderm blood-vessels are present laterad as in the preceding sections. 



Mesodermal Segments. We have seen that these are developed by the ap- 

 pearance of transverse furrows in the mesoderm (Fig. 51). Later a longitudinal 

 furrow partially separates the paired segments from the lateral unsegmented 

 mesoderm. The segments are block-like with rounded angles when viewed 



