RANA. 355 
takes 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 Froc Emsryos. 
Notochord, 
A 
Blastopore. Hypoblast. 
Neural Groove. 
Notochord. Mesoblast. 
Mesoblast. 
B 
Hypoblast. ’ 
Nerve Tube. 
Notochord., 
Ceelom., 
Mesenteron. 
Yolk Cells. 
A, Longitudinal Median Section through a Frog’s Embryo at a late Gastrula 
stage, B, Transverse Section through Frog's Embryo (after Hertwic). C, Trans- 
verse Section of Young Tadpole (ad xai.). 
The mesoblast plates divide into a dorsal vertebral plate, segmented 
into protovertebree and a ventral dateral 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. 
