52 



THE STUDY OF CHICK EMBRYOS 



Section through the Rhomboidal Sinus (Fig. 50). The neural groove is open, the 

 notochord is oval in form. The ectoderm is characterized by the columnar form of its cells. 

 At the point where the ectoderm joins the neural fold a ridge of cells projects ventrally 

 on either side. These projecting cells form the neural crests, and from them the spinal 



Neural groove 

 Ectoderm \ ^Neural crest 



_ ____ _ /Segmented zone 



Somatic mesoderm 



Splanchnic mesoderm Ccelom Notochord Entoderm Blood vessel 



FIG. 50. Transverse section through the rhomboidal sinus of a thirty-eight-hour chick embryo. 



X 90. 



ganglia are formed. The section is at the level of the segmental zone, where mesodermal 

 segments have not formed as yet. The mesodermal plates have split laterally into layers, 

 but the ccelomic cavities are mere slits. Between the splanchnic mesoderm and the ento- 

 derm blood vessels may be seen. 



Somatic mesoderm Ectoderm Primitive knot 



Codom Entoderm 



Splanchnic mesoderm 



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



embryo. X 90. 



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 undifferentiated tissue. 

 The mesoderm is split laterally into the somatic and splanchnic layers. 



Somatic mesoderm Primitive groove 



Ectoderm 



^ Ccelom 

 Splanchnopleure' / \Primitivestreak Splanchnic mesoderm 



Entoderm 

 FIG. 52. Transverse section through the primitive streak of a thirty-eight-hour chick embryo. 



X 90. 



Section through the Primitive Streak (Fig. 52). In the mid-dorsal line is the primi- 

 tive groove. The germ layers may be seen taking their origin from the undifferentiated 

 tissue of the primitive streak, beneath the primitive groove. Laterad, between the splanchnic 

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



