EMBRYO OF SEVENTEEN SEGMENTS 



49. 



by the mesoderm, which here forms the dorsal mesocardium. The ventricle hes on the right 

 side of the embryo; a few sections caudad in the series it is continuous with the ventral aorta 

 (cf. Fig. 41). Between the somatic and splanchnic mesoderm is the large pericardial cavity. 

 It surrounds the heart in this section. Dorsal to the aortae are the anterior cardinal veins, 

 which return blood from the head region. 



Transverse Section through the Caudal End of the Heart (Fig. 46).— The 

 section passes through the hind-brain. The descending aortae are separated only by a thin 

 septum which is ruptured in this section. The anterior cardinal veins are cut at the level 

 where they bend ventrad to enter the heart. The mesothehal wall of the heart is continuous 

 with the splanchnic mesoderm. On the right side of the section there is apparent fusion 

 between the myocardium of the heart and the somatic mesoderm. A pair of primitive meso- 

 dermal segments may be seen in this section lateral to the hind-brain. It may be noted here 

 that the primitive segments were not present in the sections of the head previously studied. 



Transverse Section through the Fovea Cardiaca (Fig. 47).— The descending 

 aortce now form a single vessel, the dorsal aorta, the medium septum having disappeared. The 



Ectoderm 



Mesodermal sc^mi nl 



.Neural tub 



/fVT^^AsjS^- ^Svli: V- ^ Nolochord 

 Uorsal aorta-. .r.K.'A'il.'v?;;"?;' ^r&'-s¥' A ^,«-*-*-» 



~;»v ^;■.?=iK'^■^V^■y'"^■''"v;■;■C-;i^^;v;:V.^^•;■! inal vein Cv 



— vjv_ J 7.'^ >.A"5i '.-.'■; ?"■'■..■■•■;• ..s-vJ^ ,^1;. J# 



embrvoiiic ca'lom 



^"^^^ (^ V=««;^^^^/^ / "'''''■?'''^-"'---:ipC**^'""'^---"^ ^Entoderm 



Ri^Jn^teUinc .ein splaLtn,ic\ I W =*///«c m« 



mesoderm Entoderm Fore-gut 

 Fig. 47. — TransVerse section through the fovea cardiaca of a thirty-eight-hour chick embryo. X 90. 



section passes through the entoderm at the point where it is folded dorsad and cephalad into 

 the head as the fore-gut (cf . Fig. 42)., Two sections caudad is found the opening {fovea car- 

 diacdywheie the fore-gut communicates with the fiatfened open gut 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. 48). — This section re- 

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

 vitelline vein is still large. 



Section through the Fourteenth Pair of Primitive Segments (Fig. 49) — The 

 body of the embryo is now flattened on the surface of the yolk. Here the descending aortae 

 are stiU separate and occupy the depressions lateral to the primitive segments. The section 

 is characterized by the notochord and the differentiated mesoderm which forms the primitive 

 segments, nephrotomes, somatic and splanchnic mesoderm, structures soon to be described. 

 Arising from the nephrotomes are sprout-like pronephric tubules. The tips of these hoUow 

 out and unite to form the primary excretory or mesonephric duct. 

 4 



