318 GENERAL EMBRYOLOGY 



The natural environment ordinarily varies within rather 

 narrow limits, frequently on account of the ovipositing habits 

 of the adult, and changes within these limits rarely affect the 

 course of development, so that for the subjects of heredity and 

 differentiation we should inquire here only into the question to 

 what extent external conditions are necessary factors in carry- 

 ing on the life of the organism as it exists in the form of an 

 egg or embryo. The particulars of development and heredity 

 are referable to internal characteristics which determine the 

 specific or individual quality of the reactions between organism 

 and environment. 



We may proceed, therefore, to mention a few illustrations of 

 the effects of alterations in external conditions of development, 

 not attempting to do more than to suggest the nature of the 

 work accomplished in this field; an adequate survey falls outside 

 the scope of such a text as this. (For a convenient summary, 

 see,e.^., Jenkinson, "Experimental Embryology/ 7 Oxford, 1909.) 



More is known regarding the effects upon development, of 

 chemical substances than of other conditions. While a few 

 forms, such as the minnow, Fundulus, are able to develop 

 normally in media so widely unlike, physically and chemically, 

 as sea water and distilled water, this and other forms show 

 specific effects of the presence or absence of certain salts alone. 

 Thus in Fundulus Stockard has shown that the presence of cer- 

 tain amounts of magnesium salts brings about the fusion of the 

 optic vesicle regions, so that one-eyed monsters develop, appar- 

 ently normal in other respects (Fig. 148). The eggs and em- 

 bryos of the Echinoderms offer many striking facts in this 

 connection. We have already noted that the alkalinity of the 

 sea water may determine the appearance of paternal or mater- 

 nal characters in hybrid Echinoderm larvae. Herbst and others 

 have shown that the absence of potassium salts is fatal or very 

 harmful to Echinoderm larvae, apparently on account of the 

 resulting diminution in the process of water absorption; the 

 absence of calcium causes a tendency for the blastomeres to fall 

 apart; magnesium and the sulphates are necessary for the nor- 

 mal differentiation of the alimentary tract; the production of 



