90 STAGING OF AMPHIBIAK EMBRYOS 



THE NORMAL STAGES IN THE DEVELOPMENT OF THE 

 SOUTH AFRICAN CLAWED TOAD, XENOPUS LAEVIS 



1. Unfertilized egg. Otlique view. Size 0. 9-1.0 mm. diameter. 



2. Grey crescent stage. Oblique lateral view. 



3. First cleavage completed. 

 k. Second cleavage completed. 



5. Third cleavage completed. 



6. Early cleavage stage; approximately 52 cells. 



7. Mid-cleavage stage. Detenained by easily recognizable, unpigmented rows of 



macromeres. 



8. Late cleavage; blastula stage. Determined by position of lower pigment border. 



9. Early gastrula. Oblique ventral view to show semicircular blastoporal rim. 



10. Mld-gastrula; yolk plug stage. Oblique ventral view. 



11. Late gastrula stage. Heavy pigmentation in place of yolk plug, and loss of pigment 



in presumptive neural area. Oblique ventral view. 



12. Neural plate stage. Very slight lateral compression. Dorsal view. 

 15. Neural fold stage. Slight elongation. Length 1.2 mm. 



Ik, Neural folds in contact; ciliary rotation within egg covers. Top, dorsal view. 



Bottom, frontal view, showing presumptive frontal gland and oral sucker. Length 

 1.5 mm. 



15. Neural tube stage. Top, dorsal view; eye vesicle distinct; tallbud indicated. 



Bottom, frontal view; ultimate size of frontal gland area as Indicated. Length 

 1.8 mm. 



16. Tall bud stage. Optic, otic and pronephric protuberances distinct. Total length 



2 mm. Tallbud 0.2 mm. 



17. Beginning of muscular response to mechanical stimulation. Gill plate distinct. 



Total length 2.8-5.1 mm. Tallbud O.h mm. 



18. Gill buds (two pairs) distinct; will hatch if shaken. Total length 5.7-'*.l mm. 



Tail 0.7 mm. 



19. Beginning of heart beat; can be observed under the microscope with strong illumina- 



tion. Total length 14-.9-5.2 imn. Tail 1.5 nm. 



20. Spontaneous hatching; first indications of melanin pigmentation. Total length 



5.14-5.7 mm. Head O.9 mm. Body Z.k mm. Tail 2.2 mm. 



21. Beginning of first-form tadpole stage. Nasal pits distinct; active spontaneous 



swimming begins during the phase; epidermis begins to become transparent; mouth 

 open at this time; oral sucker degenerated; cornea transparent. Total length 

 6.0-6.2 mm. Head I.5 mm. Body 1.5 inm. Tail 5-5 nm. 



22. Beginning of second-form tadpole stage. External gills disappeared; respiratory 



gulping, and feeding begin; thymus gland externally visible; lateral contour of 

 mouth is round. Total length 9 d™' slv. Head 2 mm. Body 1 mm. Tail 6 mm. 

 25. Beginning of third- form tadpole stage. Hind limb buds indicated; oral tentacle 

 buds indicated; lateral contour of mouth is wedge-shaped. Total length 50 mm. 

 av. Head 7 nm. Body 5 nun. Tail 20 mm. 



