III. 4 LIGHT 97 



H. M. VERNON. The effect of environment on the development of 

 Echinoderm larvae : an experimental enquiry into the causes of variation, 

 Phil. Trans. Boy. Soc. clxxxvi, 1895. 



E. YUNG. De 1'influence des milieux physiques sur les etres vivants, 

 Arch. Zool. Exp. et G6n. vii, 1878. 



E. YUNG. De 1'influence des lumieres colorees sur le developpement 

 des animaux, Mitt. Zool. Stat. Neapel, ii, 1881. 



5. HEAT 



As is very well known, those activities by which every 

 organism maintains its specific form can only be carried on 

 within certain definite limits of temperature. So also a certain 

 degree of heat is necessary for the due performance of the 

 functions of growth and differentiation ; above or below certain 

 limits more or less definite for each organism, but varying in 

 different organisms development is unduly accelerated or re- 

 tarded, or brought to a standstill, while its form is frequently 

 distorted as well. 



To Oscar Hertwig we are indebted for a careful inquiry into 

 the conditions of temperature under which the development of 

 the Frog's egg takes place. 



In the case of Sana fn-sca Hertwig has found the cardinal 

 temperature-points to be as follows : The normal is about 

 15-16 C. ; above this up to 20-22 C. development is accele- 

 rated without being otherwise altered ; this temperature is there- 

 fore the optimum (Fig. 51). Above this point the form of develop- 

 ment is altered, and at such a high temperature as 30 C. death 

 follows very quickly. At low temperatures (6 1 C.) there is con- 

 siderable retardation, and at the zero-point a complete cessation 

 of segmentation ; the eggs are often permanently injured. 



At the high temperatures referred to from 23 C. upwards 

 it is the yolk-cells which are primarily affected. At from 29-6 

 to 27-5 the yolk is unable to divide, though it is nucleated, and 

 segmentation is confined to the animal hemisphere, and soon 

 ceases even there (Fig. 47). At 26*5 the first furrow indeed 

 passes through the yolk, but subsequent segmentation is mero- 

 blastic, with the resulting formation of a cap of cells or blastoderm 

 lying upon and separated by a segmentation cavity from the 



JENKINSON H 



