DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO 



151 



with which it later becomes intimately associated, is called the splanch- 

 nopleure and gives rise to the lining epithelium, muscles, and blood 

 vessels of the entire mid- and hindgut. 



In between these two sheets of lateral plate mesoderm the cavity 

 is known as the primary body cavity or coelom. Eventually the 

 coelomic slit becomes continuous ventrally, from one side of the 

 embryo to the other, forming a single visceral or coelomic cavity. 

 Dorsally the junction of the lateral plates is interrupted by the noto- 

 chord and the sub-notochordal rod. This latter structure, also known 

 as the hypochordal rod, is a small rod of pigmented cells between 

 the gut roof and the notochord. Presumably this represents a vestige 

 of the connection between the two at the time of their simultaneous 

 origin at the vicinity of the dorsal lip of the blastopore. It appears first 

 at the 2.5 mm. stage. 



Origin of the Heart. The lateral plate mesoderm extends into 

 the head, ventral to the pharynx, as mesenchyme. This mesenchyme 

 becomes organized into sheets, coextensive with the more posterior 

 lateral plate sheets of somatic and splanchnic mesoderm. These will 

 give rise to parts of the heart. As the coelomic split occurs at the 

 body level, there is an extension of this split into the forming heart 

 mesoderm, which will give rise to the pericardial cavity. The outer 



Myocardium 

 Pericardial cavity 



Dorsal mesocardium 

 Pericardial cavity 

 Myocardium 

 Endocardium 

 Pericardium 



Myocardium 

 \ Endocardium 

 Pericardium 



Aortic arch 



Blood corpuscles 

 Pericardium 

 Pericardial cavity 

 Endocardium 

 Myocardium 



Development of the heart of the frog embryo. 



