EGG ACTIVATION 219 



procured by this method is small, however. In many cases an early cleavage 

 or larval stage is reached, but the advanced tadpole state or that of the fully 

 developed frog is quite rare. 



The method introduced by Bataillon is still used extensively in studies on 

 artificial parthenogenesis in the frog. Recently, Shaver ('49) finds that the 

 "second factor" is present on certain cytoplasmic granules obtained by cen- 

 trifugal fractionation. Heat at 60" C. and the enzyme, ribonuclease, destroy 

 the second-factor activity. Successful second-factor granules were obtained 

 from blood, early frog embryos, and "extracts of testis, brain, lung, muscle 

 and liver." This author also reports that heparin suppresses parthenogenetic 

 cleavage. 



In some of these parthenogenetically stimulated eggs of the frog, the diploid 

 chromosome relationships appear to be restored during early cleavage; in 

 others both diploid and triploid cells may be present. Some of these tadpoles 

 may be completely triploid (Parmenter, '33, '40). However, a large percentage 

 remain in the haploid condition (Parmenter, '33). 



A third method of approach in stimulating parthenogenetic development 

 was used by Pincus ('39) and Pincus and Shapiro ('40) on the rabbit. In 

 the former work, Pincus reports the successful birth of young from tubal 

 eggs activated by exposure to a temperature of 47° C. for three minutes. The 

 treated eggs were transplanted into the oviducts of pseudopregnant females. 

 In the latter work, eggs were exposed to a cooling temperature in vivo, that 

 is, the eggs were allowed to remain in the Fallopian tube during exposure to 

 cold. The birth of one living female was reported from such parthenogenetic 

 stimulation. 



The foregoing experiments illustrate three different procedures used on 

 three widely separated animal species, namely, changing the external chemical 

 environment of the egg, a tearing or injuring of the egg's surface followed 

 by the application of substances obtained from living tissues, and, finally, 

 changing the physical environment of the egg. To these three general ap- 

 proaches may be added that of mechanical shaking. For example, Mathews 

 ('01 ) states that mechanical shaking of the eggs of the starfish, Asterias jorbesi, 

 results in the development of a small percentage of eggs to the free-swimming 

 blastula stage. 



Some of the recent work on the initiation of development and in stimulating 

 cells to divide has emphasized the importance of cellular injury as a factor. 

 Little is known concerning the mode of action of the injuring substances. 

 Harding ('51) concludes that an acid substance is released as the result of 

 "injury" and that this acid substance causes "an increase in protoplasmic 

 viscosity and initiates cell division" in the sea-urchin egg. (Cf. theory of R. S. 

 Lillie at end of chapter.) 



That no single method has been found which activates eggs in general is 

 not surprising. The eggs of different species are not only in different states of 



