vin EFFECT OF GRAVITY ON DEVELOPMENT 403 



as he repudiated the inheritance of acquired characters he 

 must be allowed the credit of preparing the way for the 

 theories of Weismann and Nageli. His view depends on his 

 discovery that gravity has an influence on the development 

 of the embryo even in animals, since the rudiment of the 

 young frog appears, not as usual in the dark, but in the light 

 pole of the egg .when the latter is kept uppermost. Pfluger 

 proceeds therefore from the argument, that if heredity existed 

 the dark hemisphere of the frog's egg would have retained by 

 inheritance alone the function of forming the principal part 

 of the embryo. But this interpretation of the facts cannot 

 well be sustained, because as Pfliiger himself admits, the 

 whole segmentation certainly proceeds from the nucleus, and 

 the artificial inversion of the egg possibly causes a simple 

 change of position of the nucleus, and thereby of the seat of 

 its influence on the surrounding protoplasm. It is possible 

 that the protoplasm follows the nucleus in its change of posi- 

 tion. The movement of the nucleus may be due to its having 

 a lower specific gravity than the rest of the egg. Experi- 

 ments on the action of gravity would therefore be much 

 better evidence for Pfliiger's contention if gravity produced 

 the same effect after segmentation was completed. 1 



In connection with his results concerning the influence of 

 gravity on development, Pfliiger discusses recrescence, seeking 



und auf die Entwicklung des Embryo, Arch. f. die gesammte Physiologic, Bd. 

 xxxii. 1883. 



1 According to Rauber and Roux (Rauber, Schwerkraftversuche an Forellen- 

 eiern, Sitz.-Ber. Nat.-Oes. Leipzig, Feb. 1884; Roux, Ueber die Entwickl- 

 ung der Froscheier bei Aufhebung der richtenden Wirkung der Schwere, 

 Breslauer Aerztl. Zeitung, 22 Marz 1884), under the influence of a centrifugal 

 force the embryo is formed in the direction of that force, a result which 

 harmonises with the explanation I have given above. Cf. also Born ( Ueber den 

 Einfluss der Schwere auf das Froschei, Arch. f. mikrosk. Anat. Bd. xxiv. 

 1885), who formerly maintained the explanation given above, but afterwards, it 

 .seems to me on insufficient grounds, abandoned it. 0. Hertwig also ( Welchen 

 Einfiuss ubt die Schwerkraft auf die Theilung der Zellen, Jen. Zeitsch. /. 

 Naturw. Bd. xviii.), after experiments on Echinoderm ova denies any profound 

 importance to Pfluger's results. 



