66 STAGING OF AIVIPHIBIAN EMBRYOS 



b. Xenopus Laevis 



THE NORMAL STAGES IN THE DEVELOPMENT OF THE 

 SOUTH AFRICAN CLAWED TOAD, XENOPUS LAEVIS 



1. Unfertilized egg. Oblique view. Size 0. 9-1. mm. diameter. 



2. Grey crescent stage. Oblique lateral view. 



3. First cleavage completed. 



4. Second cleavage completed. 



5. Third cleavage completed. 



6. Early cleavage stage; approximately 32 cells. 



7. Mid-cleavage stage. Determined 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. Mid-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. 



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



14. 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; tailbud indicated. Bottom, frontal view; ultimate size of frontal 

 gland area as indicated. Length 1. 8 mm. 



16. Tail bud stage. Optic, otic and pronephric protuberances distinct. Total length 2 mm. Tailbud 0.2 mm. 



17. Beginning of muscular response to mechanical stimulation. Gill plate distinct. Total length 2.8-3. I mm. Tailbud 0.4 mm. 



18. Gill buds (two pairs) distinct; will hatch if shaken. Total length 3.7-4. 1 mm. Tail 0.7 mm. 



19. Beginning of heart beat; can be observed under the microscope with strong illumination. Total length 4. 9-5. 2 mm . Tail 

 1. 3 mm. 



20. Spontaneous hatching; first indications of melanin pigmentation. Total length 5.4-5.7 mm. Head 0.9 mm. Body 2.4 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. 3 mm. Body 1. 3 mm. Tail 3. 5 mm. 



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 mm. cv. Head 2 mm. Body 1 mm. Tail 6 mm. 



23. Beginning of third-form tadpole stage. Hind limb buds indicated; oral tentacle buds indicated; lateral contour of mouth is 

 wedge-shaped. Total length 30 mm. av. Head 7 mm. Body 3 mm. Tail 20 mm. 



"Not only do the body fluids of the lower forms of marine life 

 correspond exactly with sea water in their composition^ but there 

 are at least stroiig indications that the fluids of the highest animals 



are really descended from sea water the same substances 



are present in both cases, and in both cases sodium chloride pre- 

 dominates. " 



L. J. Henderson 1913: "Fitness of the Enrironment" 



"Of course, I am not forgetting thai development and evolution 

 are in the main epigenetic processes by which the more complicated 

 end stages are built upon the less complicated earlier ones, but 1 

 also refuse to forget tliat these earlier stages are also complex, that 

 the egg or the Paramecium are complex organisms and tliat develop- 

 ment is endogenetic as well as epigenetic. Both epigenesis and endo- 

 genesis are iiwolved in all developnient and evolution. " 



E. G. Conklin, 1911 



