ell 



mes 



arch 



dl. 



mes 



mes. 2 



5.C 



mes. 2. 



mes. Y. . 



FIG. 4. Sagittal sections through the frog's egg during the formation 

 and closure of the blastopore. A-D, Before rotation ; E, During rotation ; 

 F, After rotation. 



The arrow marks the egg-axis, its head the animal pole. d.L, dorsal lip ; 

 v.l., ventral lip of the blastopore ; s.c., segmentation cavity ; arch., 

 archenteron ; y.p., yolk-plug; .., intermediate zone; mes.v., mesoderm 

 iormed below the ventral lip ; mes. 2, mesoderm formed from the yolk-cells 

 pushed into the segmentation cavity. 



