EXTERNAL FACTORS 



III.; 



integration of the epithelium. Other organs were, however, 

 normally formed, the front end of the gut by invagination, the 

 notochord and mesoderm, protovertebrae, heart, pronephros, audi- 

 tory vesicles, optic vesicles, infundi- 

 bulum, and liver, until the embryo died. 

 The persistence of the yolk-plug has 

 also been induced by Gurwitsch by 

 means of halogen salts (sodium bromide 

 and lithium chloride) and weak solu- 

 tions of alkaloids (strychnine, caffein, 

 nicotine) (Fig. 63), by C. B. Wilson 

 in Rana, ChoropJiilus, and Amity si oma 

 FIG. 63. Meridional sec- by means of sodium chloride and 

 tion through a lithium Ringer's solution, and by Morgan with 

 gastrula of Bufo nugaris. . ,.., . ,. , -r, , .,, 



fc, blastocoel. (After Gur- various lithium salts; and Bataillon, 



witsch, from Korschelt and who has used isotonic solutions of cane- 

 sugar, sodium chloride, and a large 



number of other salts for the purpose, claims that in this case 

 the results produced depend upon the osmotic pressure alone, and 

 are therefore due to a withdrawal of water from the developing 

 embryo. 



FIG. 64. Sections of Frogs' eggs grown in solutions of, A, ammonium 

 iodide (1-5%), and, B, urea (2-3%). In both cases segmentation is inero- 

 blastic, although in A there are a few large divisions in the yolk. In 

 B the inultinucleate cell masses of the animal hemisphere protrude 

 above the surface. The nuclei are large, lobed, and homogeneously 

 chromatic in both cases. (Ammonia is probably present in the solution 

 of urea.) 



Recent experiments made by the author do not, however, bear 

 out this conclusion. In the first place, it is to be observed that 

 isotonic solutions (isotonic with a 0-625 % NaCl solution) do not 



