EARLY DEVELOPMENT IN THE FROG. 313 



fr 



a quarter of a century ago. Among the early papers on the 

 subject may be mentioned those of O. Hertwig (1892, 1894 

 1895, 1896); Gurwitsch (1895, 1896); Morgan and Tsuda (1894). 



These and other workers have studied the early development 

 of certain teleosts, amphibia and birds under the influence of 

 such physical factors as: gravity, mechanical pressure, tempera- 

 ture, light, electricity and magnetism, X-rays, radium emana- 

 tions, atmospheric pressure, and a wide variety of chemical 

 substances. 



It is perhaps sufficient, here, merely to mention the essential 

 similarity of terata produced by widely different methods, such, 

 e.g., as high and low temperatures, many different chemical 

 agents, and heterogeneous hybridization. Equatorial gastrula- 

 tion, embryos with permanent yolk plugs, with spina bifida, 

 microcephalic forms and many other abnormalities in the frog 

 have been produced by nearly all the methods known to experi- 

 mental teratology. For instance, microcephalic frog embryos 

 with permanent yolk plugs, and spina bifida have been produced 

 by exposing different stages in development to such external 

 agents as: high temperature (Jenkinson, 1909, p. 98, fig. 49); 

 0.6 per cent. NaCl (Hertwig, 1894, p. 315, 316, 1895, plate XX., 

 figs. 5, 6, 7, 10, 12, 15, 16, et al) ; fertilizing "over-ripe" eggs 

 (Hertwig, 1892); strychnine (Gurwitsch, 1896); sugar solutions 

 (Bataillon, 1901, figs. 2, 5); Jenkinson (1906) produced these 

 same types (among many others) in a number of isotonic solutions 

 of Na 2 SO 4 , K 2 SO 4 , NaNO 3 , urea, sugar solutions, et al. 



Cyclopia in Fundulus has been produced by physical and 

 chemical agents and by hybridization. Stockard (1909) obtained 

 this anomaly with the aid of magnesium chloride and for a time 

 ascribed a specific cyclopia producing property to this salt. 

 Later, Stockard produced cyclopic monsters with alcohol and 

 other substances and was forced to abandon his theory of 

 specificity. McClendon (1912) had no difficulty in producing 

 cyclopia in isotonic solutions of NaCl, LiCl, MgCl 2 , NaOH and 

 in alcohol, and other substances. McClendon is somewhat at a 

 loss to explain his results but sums up the facts in these words: 

 "We need only suppose that the cells between the eye anlagen 

 are more easily affected than other cells of the embryo, to explain 



