CHICK EMBRYO OF EIGHTEEN PRIMITIVE SEGMENTS 



61 



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

 40). — 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- 



Ectoderm 



Neural tube 



iNeurai Cresf 



Segmental zone 



•Somaitc mesoderm 



Splanchnic mesoderm „ . I ... , I \ , ^ Blood vessel 



Coe/om' /Votochord' \Entoderm 



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



Ectoderm 



Somatic mesoderm 



Primitive node 



Coe/om / 



Entoderm I Splanchnic mesoderm 



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



X9°- 



/ 



Somatic mesoderm Primitive groove 



Edoderm 



I 



\ Coelom 

 Entoderm / Primitive streak Splanehnie mes. 



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



SptanchnopUure ' 



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 



