DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO 



145 



Origin of the Somites. The mesoderm posterior to the pharynx 

 and lateral to the notochord (as seen in transverse section) taices 

 an inverted horseshoe shape around the archenteron and the yolk. 

 The uppermost level of this mesoderm, lateral to the nerve and noto- 



Stomodeal clert 



Pericardium 

 Myocardium 

 Endocardium 



Thyroid gland 

 Mesenchyme 



Bulbus arteriosus 



Pericardial cavity 



Atrium 



Sinus venosus 



Right vitelline vein 



Liver diverticulum 



Coelomic spaces 



Yolk 



Splanchnic mesoderm 



Somatic mesoderm 



r 



Proctodeum 



Frontal ( horizontal) section of the 7 mm. 

 frog larva at the level of the developing heart. 

 (Redrawn and modified after Huettner.) 



chord, is termed the segmental or vertebral plate and the more ventral 

 portion of the same sheet of mesoderm is called the lateral plate meso- 

 derm. This lateral plate extends continuously in a ventral direction 

 and surrounds the yolk, just within the belly ectoderm. Shortly, all 

 of this mesoderm will be separated off into a dorsal somite mass 



