]24 



THEORIES OF INHERITANCE AND DEVELOPMENT 



cially the service of Loeb, Herbst, and Driesch to show how essential 

 a part is played by the environment in the development of specific 

 organic forms. The limits of this work will not admit of any adequate 

 review of the vast array of known facts in this field, for which the 

 reader is referred to the works especially of Herbst. I shall only 

 consider one or two cases which may serve to bring out the general 

 principle that they involve. Every living organism at every stage 



of its existence reacts to its envn-on- 

 ment by physiological and morpho- 

 logical changes. The developing 

 embryo, like the adult, is a moving 

 equilibrium — a product of the response 

 of the inherited organization to the 

 external stimuli working upon it. If 

 these stimuli be altered, development 

 is altered. This is beautifully shown 

 by the experiments of Herbst and 

 others on the development of sea- 

 urchins. Pouchet and Chabry showed 

 that if the embryos of these animals 

 be made to develop in sea-water con- 

 taining no lime-salts, the larva fails to 

 develop not only its calcareous skele- 

 ton, but also its ciliated arms, and a 

 larva thus results that resembles in 

 some particulars an entirely different 

 specific form ; namely, the Tornaria 

 larva of Balanoglossns. This result 

 is not due simply to the lack of neces- 

 sary material; for Herbst showed 

 that the same result is attained if a 

 slight excess of potassium chloride be 

 added to sea-water containing the nor- 

 mal amount of lime (Fig. I40- Ii"^ 

 the latter case the specific metabolism 

 of the protoplasm is altered by a particular chemical stimulus, and a 



new form results. 



The changes thus caused by slight chemical alterations m the 

 water may be still more profound. Herbst ('92) observed, for ex- 

 ample, that when the water contains a very small percentage of 

 lithium chloride, the blastula of sea-urchins fails to invagmate to 

 form a typical gastrula, but evaghiatcs to form an hour-glass-shaped 

 larva, one half of which represents the archenteron, the other half 

 the ectoblast. Moreover, a much larger number of the blastula-cells 



Fig. 141. — Normal and modified 

 larvae of sea-urchins. [HERBST.] 



A. Normal Pluteus {Strongyloce7itro- 

 tus). B. Larva {Spkcsreckhiits) at the 

 same stage as the foregoing, developed 

 in sea-water containing a slight excess 

 of potassium chloride. 



