EMBRYO OF SEVENTEEN SEGMENTS 



SI 



Transverse Section through the Primitive (Hensen's) Knot or Node (Fig. 51). 

 — The section shows the three germ layers fused inseparably at the "knot" into a mass of 



Somatic mesoderm Ectoderm 



Primitive knot 



CcBlom Entoderm Splanchnic mesoderm 



Fig. 51. — Transverse section through the primitive (Hensen's) knot of a thirty-eight-hour chick embryo. 



X90. 



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

 layers. 



Transverse Section through the Primitive Streak (Fig. 52). — In the mid-dorsal 

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



Somatic mesoderm 



Primitive groove 



Fxlodi I If! 



^•;vS^^^^?;25^-' 



..•/•ft r.:;.,-.trV.!= 



Primilive streak Splamhinc mesoderm 



Splanclnwpleure'^ 



Entoderm 

 Fig. 52. — Transverse section through the primitive streak of a thirty-eight-hour chick embryo. X 90. 



ferentiated 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 t hese are developed by th e ap- 

 pearanc e of trans jvers e furrows in the mesoderm (Fig. 53) . Later a longitudinal 

 furrow partially separates the paired segments from the lateral unsegmented 

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

 dorsally, triangular in transverse sections (Figs. 49 and 53). They are formed 

 cranio-caudally, the most cephalad being the first to appear. The first four lie 

 in the head region. The segments contain no definite cavity but a potential 

 cavity representing a portion of the coelom is filled with cells, and the other cells 

 of the segments form a thick mesothelial layer about them (Fig. 49) . The ventral 

 wall and a portion of the median wall of each primitive segment become trans- 

 formed into mesenchyma w hich surrounds the neural tube and notochord (Fig. 

 290). The remaining portion of the segments persist as the dermo-muscular plates. 

 The cells of the mesial portions of the plates, the myotomes , elongate and give rise 



