38 



Heredity and Environment 





a B C J) E b JF 9 



Fig. 15. Distribution of Bacteria in the Spectrum. The largest 

 group is in the ultra-red at the left ; the next largest group is in the yellow- 

 orange close to the line D. (From Jennings, after Engelmann.) 



4, ec). It is an interesting fact that the same sort of response 

 follows when a frog's egg is pricked hy a needle, thus showing 

 that in this case the egg does not distinguish between the prick of 

 the needle and that of the spermatozoon. The spermatozoon is 

 usually a locomotor cell and it responds differently to certain 

 stimuli, just as many bacteria and protozoa do; spermatozoa are 

 strongly stimulated by weak alkali and alcohol, they gather in 

 certain chemical substances and not in others, they collect in great 

 numbers around fertilizable egg cells, etc. 



Sensitivity of Oosperm and of Embryo. — The movements of 

 fertilized egg cells, cleavage cells, and early embryonic cells are 

 usually limited to flowing movements within the individual cells. 

 These movements, which are of a complicated nature, are of the 

 greatest significance in the differentiation of the egg into the 

 embryo ; they, are caused chiefly by internal stimuli and by non- 

 localized external ones. Modifications of the external stimuli 

 often lead to modifications of these intracellular movements and 

 to abnormal types of cleavage and development — in short, these 

 movements show that the fertilized egg is differentially sensitive. 



In the further course of development particular portions of the 

 embryo become especially sensitive to some kinds of stimuli, while 

 other portions become sensitive to others. In this way the differ- 

 ent sense organs, each especially sensitive to one particular kind 

 of stimulus, arise from the generalized sensitivity of the oosperm, 

 and thus general sensitivity, which is a property of all protoplasm, 



