RANA. 



355 



lakes place from before backwards, hence at the embryonic rim the 

 epiblast, mesoblast and hypoblast are all merged into one common 

 mass of cells. 



Fig. 253. — Sections of Frog Embryos. 



Notochord. , 



Blastopore. 



Notochord 

 Mesoblast. 



Hypoblast. 



Mesoblast. 



Nerve Tube. 

 Notochord. 



Coelom.,, 



A, Longitudinal Median Section through a Frog's Embryo at a late Gastrula 

 stage. B, Transverse Section through Frog's Embryo (after Hertwig). C, Trans- 

 verse Section of Young Tadpole (adnat,). 



The mesoblast plates divide into a dorsal vertebral plate, segmented 

 into protovertebrse and a ventral lateral plate. The lateral plates grow 

 downwards on either side, and a coelomic cavity arises in each by a 

 splitting between the cells, the mesoblast then forming an outer somatic 

 layer and an inner splanchnic. 



