98 



EXTERNAL FACTORS 



III. 5 



nucleated yolk. The eggs then die. At lower temperatures 

 25-23 the yolk is also affected, and many eggs die in the 

 ' morula ' stage ; such as do survive give rise to distortions or 

 monstrosities (Figs. 48, 49). The injury to the yolk interferes 



Fio. 47. Meridional sections of eggB of Sana fusca developed (A) at 

 29-5 C-, (B) 26-5 C. Five hours ten minutes after fertilization, k, nuclei ; 

 p, pigment. 



FIG. 48. Meridional sections of eggs of Kana fusca developed at 

 26-5 C. One day after fertilization. /.-, nuclei ; kh, blastocoel ; z, cells 

 imbedded in unsegmented yolk. 



FIG. 49. Abnormal embryos of Rana fusca, produced by heat. A, embryo 

 two days old developed at a temperature of 24 C. ; B, embryo three days 

 old, reared at a temperature of about 25 C. lr, brain ; y.p, yolk-plug ; 

 t .1, tail-bud ; t, tail ; s, sucker ; g, gill. 



