THE 48-HOUR EMBRYO 



221 



buds extend forward along and between the vitelline veins. The foregut at 

 this level is surrounded by a thick layer of splanchnic mesoderm which, 

 together with a thinner layer of somatic mesoderm, encloses a true embryonic 

 coelom. Above the gut the two dorsal aortae have fused in the midline. The 

 posterior cardinal veins run in the mesenchyme on each side of the embryonic 

 coelom. Ventral to the two vitelline veins is a section through the ventricle 

 of the heart. 



Posterior to the liver buds the gut opens out into the yolk, as shown in 

 Figure 141. More posteriorly the endoderm becomes a thin sheet of cells 

 closely applied to the overlying mesoderm (Fig. 142). Differentiation of the 

 somites, nephrotome, and lateral mesoderm is clearly seen. The sclerotome 

 is spreading toward the notochord, and a clear plate of dermatome is seen. 

 The myotome, which is not very well defined, is between the dermatome and 

 the sclerotome. The nephrotome has formed mesonephric ducts and some 

 tubules. The lateral mesoderm encloses an embryonic coelom and an extra- 

 embryonic coelom. At this level of the body paired dorsal aortae are present. 

 In Figure 143, branches of the dorsal aortae pass out into the splanchnopleure 

 over the yolk as the vitelline arteries. 



EMBRYONIC COELOM 



NEURAL TUBE 



NOTOCHORD 



Fig. 141. Opening of the midgut. 



LATERAL MESODERM 

 NEPHROTOME 



DORSAL AORTA 



Fig. 142. The somites and mesonephros. 



