III. i ^ GRAVITATION 83 



original, now inclined, and the actual axis, or vertical line of 

 intersection of the first two furrows. While, therefore, the 

 cleavage planes are definitely related to the vertical bufc not to 

 the original axis of the egg, the median plane of the embryo 

 appears to be jointly determined by both. 



From these experiments Pfliiger drew remarkable and far- 

 reaching conclusions. He conceived of the eggas being meridionally 

 polarized, composed of a large number of rows of molecules placed 

 meridionally with regard to the original egg-axis. Each row is 

 equivalent developmentally to every other row, but within the 

 limits of each the molecules are of different value, since one end, 

 for example, is anterior, the other posterior. Which of these 

 equipotential rows shall lie in the sagittal plane of the embryo is 

 decided by gravity, and by gravity alone. Similarly the vertical 

 direction of the first two furrows, the horizontal direction of 

 the third, is due to the operation of some general, though at 

 present unknown, law, in accordance with which ' die Schwerkraft 

 die Organisation beherrscht '. l 



The original structure of the egg, on the other hand, has no 

 definite relation either to segmentation or to the symmetry of 

 the embryo, except, of course, in so far as the original axis 

 together with the actual vertical axis determines its sagittal 

 plane, the white side its dorsal side. 



' . . . das befruchtete Ei gar keine wesentliche Beziehung zu 

 der spateren Organisation des Thieres besitzt, sowenig als die 

 Schneeflocke in einer wesentlichen Beziehung zu der Grosse 

 und Gestalt der Lawine steht, die unter Umstanden aus ihr sich 

 entwickelt. Dass aus dem Keime immer dasselbe entsteht, 

 kommt daher, dass er immer unter dieselben ausseren Beding- 

 ungen gebracht ist/ 2 



It will certainly be agreed that so sweeping and revolutionary 

 a dogma as this is in need of very substantial support ; and 

 though the facts as stated by Pfliiger are incontrovertible, as 

 the repetition of his experiments has shown, it is unfortunate 

 that he did not also take into consideration the internal changes 

 occurring in his forcibly inverted eggs. The deficiency has been 

 made good by Born. Like Pfliiger, Born found that in the 

 1 1. c. infi-a, xxxii. p. 24. 2 1. c. infra, xxxii. p. 64. 



G 2 



