100 EXTERNAL FACTORS III. 5 



Morgan has similarly found that the fertilized (not, however, 

 the unfertilized) eggs of an American species (/?. palustris] which 

 have been subjected to a temperature of 1 C. and then allowed 

 to develop under normal circumstances exhibit spina bifida and 

 persistent yolk-plug. 



Schulze has also observed these abnormalities as the result of 

 excessive cold. On one point, however, this author is not quite 

 in agreement with O. Hertwig, for he states that eggs and 

 embryos exposed to in various stages do continue to develop, 

 though of course very slowly. Thus, in the case of eggs exposed 

 shortly after fertilization, the blastula stage was only reached in 

 ten days, while a month elapsed before the blastopore was formed. 

 Lillie and Knowltoh, however, state that in R. virescens and 

 Amblystoma tigrinum segmentation is totally inhibited at 0. 

 In another species of Frog (R. esculenta) which spawns much 

 later in the year in May and June the cardinal points were 

 found by Hertwig to be much higher, and the eggs endured 

 a temperature of 33 C. without injury. They are, in fact, 

 acclimatized to a higher temperature, and it is very interesting 

 to notice that Davenport and Castle have succeeded in artificially 

 acclimatizing the eggs of another Amphibian (Bufo lentiginosus) 

 to a considerable degree of heat. Eggs were reared at 15 C. 

 and 24-25C. After four weeks the heat rigor temperature 

 was 40 C. for the former, 43-2 for the latter ; and in another 

 experiment the temperature was raised to 43-5 by allowing the 

 eggs to develop at 33-34 for seventeen days. 



A similar lowering of the minimum seems to have been observed 

 by Lillie and Knowlton in the case of Amblystoma tigrinum. In 

 this form, which spawns much earlier than Rana virescens } there 

 is considerably less retardation of development at 4. 



That temperature markedly influences the rate of development, 

 or, as Hertwig puts it, that the quantity of developmental work 

 performed per unit of time is a function of the temperature, is 

 abundantly clear, and is well shown in the annexed diagrams 

 (from Hertwig), in which the curves show the times taken to 

 reach various stages at various temperatures (Figs. 52, 53). 



It will be seen that as the temperature sinks the rate of 

 development, or rather of differentiation, decreases, but at an 



