DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO 



149 



somite cells is thicker and will give rise to the skeletal muscles of 

 the back and body. This portion is known as the myotome. A few 



Notochord 

 Neurocoel / Dorsal aorta 



Aortic orches 



Internal corotid 

 artery 



Venous circulation 



in liver Liver diverticulum 



External 

 carotid artery 

 Bulbus arteriosus 

 Ventricle 

 Atrium 

 Sinus venosus 

 Right vitelline vein 



Visceral orches I and II 



Sections of 

 external gills 



Liver diverticulum 



The earliest complete, closed blood vascular system 

 of the frog embryo (found at the 4 mm. stage). 

 Schematized drawing. 



scattered cells may be seen between the myotome and the notochord, 

 proliferating off from the somite. These are mesenchymal cells known 

 as the sclerotome. They will give 

 rise to the vertebral skeleton. 



Origin of the Excretory Sys- 

 tem. The mesomere and hypomere 

 are still connected. The lateral 

 plate (i.e., mesomere), separate 

 from the epimere (somite), now 

 develops along its dorsal border a 

 continuous, antero-posterior band 

 of mesoderm known as the nephro- 

 tome. This will give rise to parts 

 of the larval and the adult ex- 

 cretory systems. The nephrotomal 

 band enlarges in a lateral direction 

 but maintains cellular connections 

 with the remaining dorsal limit of 

 the lateral plate mesoderm. The 

 influence of the segmentation of 

 the vertebral plate extends to the 



separated nephrotomal band, dividing it also into a series of meta- 

 meric nephrotomes. This nephrotomal segmentation is transitory in 



THE 7 MM. 

 FROG TADPOLE 



Frontal section through the level of 

 the heart of the 7 mm. tadpole. 



