78 STAGING OF AMPHIBIAN EMBRYOS 



The following plates illustrate the various types of abnormalities encountered in 

 the development of Bana plpiena. These abnormalities may be caused by environmental fac- 

 tors such as extremes of temperature, lack of oxygen, toxic substances in the environment- 

 al medium, pressure, or variations in osmotic pressure. There are also internal factors 

 which may cause such abnormalities, such as the age of the egg at the time of insemina- 

 tion, incompatability of the sperm and egg nuclei, etc. The student is advised to become 

 acquainted with all of these types so that he will recognize them in his experimental 

 work. A brief description of each of the types follows: 



1. Depigmentation of the animal pole in vicinity of germinal vesicle common 

 in eggs aged in the uterus. 



2. Depigmentation in one blastomere, does not affect lines of tension in 

 cleavage . 



5- Eccentric cleavage, leaving depigmented area in larger blastomere. 



k. Cytolysls plus depigmentation in one of two blastomeres, preceded cleav- 

 age. 



5. Apparently normal cleavage but development will not progress far with the 

 pigment scattered. 



6. Severe cytolysls during earliest cleavages. 



7. Third cleavage achieved even with depigmentation areas. 



8. Pigment migration into cleavage furrows in aged egg. 



9. Blastula cells with uneven distribution of pigment - aged egg. 



10. Reversal of animal-vegetal pole pigmentation in aged egg blastula. 



11. Typical cytolysls during the earliest cleavages - no furrows. 



12. Late blastula going to pieces in attempting to gastrulate (typical of in- 

 compatible hybrids such as Bullfrog sperm and Leopard frog egg). 



13. Same as #12, view of region where blastoporal lips would occur in normal 

 egg. 



lU. - 16. Impeded invagination results in exogastrula and often the anterior 

 region of the future embryo Is pulled Inward, leaving the curious ring 

 formation. 



17. Gastrulation achieved, but lips abnormal. 



18. Pigment in large yolk plug cells, abnormal gastmlatlon. 



19. Yolk plug stage but viability very low in eggs where the pigment is so 

 scattered. 



20. Curious reversal of pigment with the yolk plug containing practically all 

 pigment and the presumptive epidermis being grey to yellow. 



