XIV 



THE INFLUENCE OF THE ENVIRONMENT 

 AMONG ANIMALS AND PLANTS 



1. A BRIEF inquiry into the part played by the environment 

 among animals and plants forms a necessary introduction to 

 a similar inquiry as regards man such as we shall undertake 

 in the next chapter. 



In the second chapter something was said as to fertilization 

 and as to the development of the zygote or fertihzed egg to the 

 adult form. It has been shown by Herbst that, if this process 

 of development is to result in a normal adult, all the factors 

 comprising the normal environment must be present. Herbst 

 ascertained the exact composition of sea-water at Naples.^ Using 

 as his material the larvae of sea-urchins, he changed slightly in 

 many different ways the composition of the water. The experi- 

 ments were very exhaustive. At every stage in the development 

 he observed the results of abstracting one or more of the con- 

 stituents of sea-water — the normal environment of the larvae. 

 Commenting upon the results of these experiments, Jenkinson 

 says : * Whatever may be the ultimate explanation of the facts, 

 there can be no doubt whatever that the most complete demonstra- 

 tion has been given of the absolute necessity of many of the 

 elements occurring in ordinary sea-water, its normal environment, 

 for the proper growth and differentiation of the larva of the sea- 

 urchin. Nor is this all. Some of the substances are necessary 

 for one part or phase of development, some for another, some 

 at the very beginning, others only later on. Thus potassium, 

 magnesium and some degree of alkalinity are essential for 

 fertihzation, chlorine and sodium for segmentation, calcium for 

 the adequate cohesion of the blastomeres, potassium, calcium 

 and the hydroxyl-ion for securing the internal osmotic pressure 

 necessary for growth, while without the sulph-ion and magnesium 

 the due differentiation of the alimentary tract and the proper 



' These experiments have been summarized by Jenkinson {Experimental Embryo- 

 logy, pp. 141 £f.). 



