76 STAGING OF AMPHIBIAN EMBRYOS 



The following plates illustrate the various types of abnormalities encountered in the de- 

 velopment of Rana pipiens. These abnormalities may be caused by environmental fac- 

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

 mental 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 

 insemination, 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 exper- 

 imental 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. 



3. Eccentric cleavage, leaving depigmented area in larger blastomere. 



4. Cytolysis plus depigmentation in one of two blastomeres, preceded cleavage. 



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



6. Severe cytolysis during earliest cleavages. 



7. Third cleavage achieved even with depigmentation areas. 



8. Pigment migration into cleavage furrows in aged egg. 



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



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



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



12. Late blastula going to pieces in attempting to gastrulate (typical of incompatible hybrids such as Bullfrog 

 sperm and Leopard frog egg). 



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



14. - 16. Impeded invagination results in exogastrula and often the anterior region of the future embryo is 



pulled inward, leaving the curious ring formation. 



17. Gastnilation achieved, but lips abnormal. 



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



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. 



"About the fourth day the egg beginneth to step from the life of a 

 plant to tliat of an animal. From tlial to the tenth it enjoys a sensitive 

 and moving soul as Animals do, and after that it is completed by degrees 

 and adorned with Plume, Bill, Claives, and other furniture it liastens to 

 get out. — For all animals resemble one or other of those above men- 

 tioned (fowl, goose, duck, pigeon, frog, serpent, fish, crustacean, 

 silkworms, sheep, goats, dogs, cats, deer, oxen, man) and agree with 

 them either generally or specifically, and are procreated in the same 

 manner, or the mode of their generation at least is referrable by 



analogy to that of one or other of them. Before man attains maturity, 



he was a body, an infant, an embryo. And then it is indispensable to 

 inquire further as to what he was in his mother's womb before he was 

 an embryo or fetus. — Nature, by steps which are the same in the 

 formation of any animal whatsoever, goes through the forms of all 

 animals, as I might say egg, worm, embryo, and acquires perfection 

 with each step. " 



William Harvev, 1578-1657 



