428 



INHERITANCE AND DEVELOPMENT 



H. The External Conditions of Development 



We have thus far considered only the internal conditions of devel- 

 opment which are progressively created by the germ-cell itself. We 

 must now briefly glance at the external conditions afforded by the 

 environment of the embryo. That development is conditioned by 

 the external environment is obvious. But we have only recently 



come to realize how intimate the rela- 

 tion is ; and it has been especially 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 re- 

 ferred 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 liv- 

 ing organism at every stage of its exist- 

 ence reacts to its environment by physio- 

 logical and morphological 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 



A. Normal Piuteus {stro.gyiocen- ^^d Chabry showed that if the embryos 

 trotus). B. Larva {spk(Erechinus) at of thcsc animals be made to develop in 

 the same stage as the foregoing, devei- ^ea-watcr Containing no limc-salts, the 



oped m sea-water contammg a slight _ t> ' 



excess of potassium chloride. larva fails to develop not Only its calca- 



reous skeleton, but also its ciliated arms, 

 and a larva thus results that resembles in some particulars an entirely 

 different specific form ; namely, the Toniaria larva of Balanoglossns. 

 This result is not due simply to the lack of necessary material ; for 

 Herbst showed that the same result is attained if a shght excess of 

 potassium chloride be added to sea-water containing the normal 

 amount of lime (Fig. 193). In the latter case the specific metabolism 

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

 new form results. 



Fig. 193. — Normal and modified 

 larvEe of sea-urchins. [Herbst.] 



