44 



SEGMENTATION OF THE FERTILIZED OVUM 



appears a shaded thickening, the primitive knot or node (Hensen's). From 

 this point it grows cranially, forming along the midline a thicker layer of 

 tissue, the notochordal plate or head process (Fig. 26). At twenty-five hours 

 (Fig. 31), the mesoderm forms lateral wings which extend cephalad beyond 

 the limits of the area pellucida. The space between these wings is the 

 proamniotic area. A transverse section through the primitive streak at 

 twenty hours (see guide line C, Fig. 26) shows the three germ layers distinct 



Ected. 



Neural plate 



Mesoderm 



Notochordal plate "Entoderm 



Ectode 



Primitive node 



Entoderm 



Mesoderm 



Ectoderm 



im'itjve' Streak 



■Entoderm 

 /vlesoaerm 



Fig. 28. — Transverse sections through the embryonic area of a twenty-hour chick. A, through the head 

 process; B, through the primitive node; C, through the primitive streak. X 165. 



laterally (Fig. 28 C). In the midline, a depression in the ectoderm is the 

 primitive groove. In this region there is no line of demarcation between 

 ectoderm and mesoderm. A transverse section through the primitive node (Fig. 

 28 B, guide line B, Fig. 26) shows in this region the marked proliferation of cells, 

 which are growing cephalad to form the notochordal plate (head process). 



A transverse section through the notochordal plate just beginning to form 

 at this stage (Fig. 28 A, guide line A, Fig. 26) shows the thickening near the 

 midline which will separate from the lateral mesoderm and form the notochord. 



