5° 



THE STUDY OF CHICK EMBRYOS 



Transverse 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 ven- 



Neurai tube 



Mesodermal segment 



Neural cavity 



Ectoderm 

 Notochord 



Coelom 

 Splanchnic mesoderm 



R. vitelline vein. 



Dorsal aorta 

 Somatopleure 



S planchnopleure Open gut Entoderm 



Fig. 48. — Transverse section caudal to the fovea cardiaca of a thirty-eight-hour chick embryo. X 90. 



Neural tube 



Mesodermal segment 

 Central cells of segment. 



Somatic, mesoderm' 



Ectoderm 



Pronephric tubule 



S-blaiichnic mesoderm 



Descending aorta' I Entoderm 



Notochord 



Ccelom 



Fig. 49.— Transverse section through the fourteenth pair of mesodermal segments of a thirty-eight-hour 



chick embryo. X 90. 



Neural groove 



Ectoderm 



Somatic mesoderm 



Neural crest 



Segmental zone 



Splanchnic mesoderm Cwlom Notochord Entoderm Blood vessel 



Fig. so.— Transverse section through the rhomboidal sinus of a tliirty-eight-hour diick embryo. X 90 



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

 ganglia are formed. The mesodermal plates have spUt laterally into layers, but the coelomic 

 cavities are mere slits. Between the splanchnic mesoderm and the entoderm blood vessels 

 may be seen. 



