102 



EXTERNAL FACTORS 



III. 5 



for the tadpoles of Eana virescens and Bufo lentiginosus. The 

 same authors state that at low temperatures (below 3 in the 

 case of the Frog, below 6 in the case of the Toad) growth was 

 altogether inhibited, while at 2 there was an actual shortening 

 in length in the case of the Frog tadpole, due, it is suggested, 

 to a diminution in the turgor of the cells. 



The cardinal points have also been determined for the Hen's 

 egg. According to Kaestner normal development occurs only 



FIG. 53. Effect of temperature upon the growth of the tadpole of the 

 frog (Eana fusca). A, B, developed at a temperature of 14-5-15 C. ; 

 A, two days old, circular blastopore (Stage I in Fig. 52) ; B, three days 

 old (Stage II in Fig. 52) ; C, D, developed at a temperature of 20 C. ; 

 C, three days old (Stage V in Fig. 52) ; D, four days old (Stage VI in 

 Fig. 52). (From Minot, 1907, after 0. Hertwig, 1898.) 



between 95 and 102 F. (35 and 39 C.). The maximum, the 

 temperature above which the embryo dies, is 43 C. ; the mini- 

 mum, at which development stands still, 28 C. Edwards, how- 

 ever, fixes the minimum or physiological zero at 20-21 C., for, 

 as the annexed diagram shows(Fig. 54),development may continue 

 between 20 and 29, though it is, of course, very much retarded. 

 Edwards has further made the highly interesting observation 



