The cell in development and inheritance . pment isaltered. This is beautifully shown by theexperiments of Herbst and others on thedevelopment of sea-urchins. PouchetA. Normal Piuteus istrongyiocen- ^nd Chabry showcd that if the cmbryostrotus). B. Larva {sphcsreckinus) at of thesc animals be made to develop inthe same stage as the foregoing, devei- gea-watcr Containing no limc-salts, the oped m sea-water contammg a slight . ° 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 particu
The cell in development and inheritance . pment isaltered. This is beautifully shown by theexperiments of Herbst and others on thedevelopment of sea-urchins. PouchetA. Normal Piuteus istrongyiocen- ^nd Chabry showcd that if the cmbryostrotus). B. Larva {sphcsreckinus) at of thesc animals be made to develop inthe same stage as the foregoing, devei- gea-watcr Containing no limc-salts, the oped m sea-water contammg a slight . ° 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 entirelydifferent specific form; namely, the Toimaria larva of result is not due simply to the lack of necessary material; forHerbst showed that the same result is attained if a slight excess ofpotassium chloride be added to sea-water containing the normalamount of lime (Fig. 193). In the latter case the specific metabolismof the protoplasm is altered by a particular chemical stimulus, and anew form Fig. 193. — Normal and modifiedlarvae of sea-urchins. [HERBST.] THE EXTERNAL CONDITIONS OF DEVELOPMENT 429 The changes thus caused by sHght chemical alterations in thewater may be still more profound. Herbst (92) observed, forexample, that when the water contains a very small percentage oflithium chloride, the blastula of sea-urchins fails to invaginate toform a typical gastrula, but evaginates to form an hour-glass-shaped Fig. 194. — Regeneration in coelenterates [A, B, from ; C, D, from BiCKFORD). A. Polyp (C(V-/t7;), producing new tentacles from the aboi-a/ side of a lateral Hydroid ( J/c/vz/.r/vV?), generating a head at each end of a fragment of the stem suspended inwater, C. D. Similar generation of heads at both ends of short pieces of the stem, in Titbularia. larva, one half of which represents the archenteron, the other halfthe ectoblast. Moreover, a much larger number of the blastula-cellsundergo the differen
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