6o 



THE STUDY OF CHICK EMBRYOS 



head as the fore-gut. The cavity is the fovea cardiaca and two sections caudad it communicates 

 with the flattened space between the entoderm and the yolk. On each side of the fore-gut are 

 the large vitelline veins, sectioned obliquely. As the splanchnic mesoderm overlies these veins 

 dorsad, it is pressed by them on each side against the somatic mesoderm and the cavity of the 

 ccelom is thus interrupted. 



Transverse Section Caudal to the Fovea Cardiaca (Fig. 46). — This section re- 

 sembles the preceding save that the primitive gut is without a ventral wall. The vitelline vein 

 on the left is still large. 



Neural lube .Neural Cavity 



Ectoderm 

 Mes. segment J^SiS&S B&Jk Jlatochord 



Fig. 



\ Entoderm 

 Jplanchnopkurk Open cjut 

 46. — Transverse section caudal to the fovea cardiaca of a thirty-six-hour chick embryo. 



Xoo. 



Mes. segment- 

 Central celt, 

 of segment 



•Scm. mesoderm 



NeuraJ tube 



Ectoderm 



Mesonephr'ic duct 



Splanchnic, , 

 mesoderm 



Coelom 



Descending .. 



aorta /Votichord, 



Entoderm 



Fig. 47. — Transverse section through the fourteenth pair of mesodermal segments of a thirty-six-hour 



chick embryo. X 90. 



Section through the Fourteenth Pair of Primitive Segments (Fig. 47). — The 

 body of the embryo is now flattened on the surface of the yolk. The dorsal aortae have sepa- 

 rated and occupy the depressions lateral to the primitive segments. The section is characterized 

 by the differentiated mesoderm which forms the primitive segments, nephrotomes, somatic and 

 splanchnic mesoderm, structures soon to be described. 



Transverse Section through the Rhomboidal Sinus (Fig. 48).— 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 crest of cells projects ventrally 

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

 are formed. The mesodermal plates have split laterally into layers, but the ccelomic cavities 

 are mere slits. Between the splanchnic mesoderm and the entoderm blood-vessels may be seen. 



