FOUNDATION OP A THEORY OF HEREDITY. 177 



organization of the animal subsequently formed by it, and that it 

 is only the recurrence of the same external conditions which 

 causes the germ-cell to develope always in the same manner. The 

 force of gravity was the first factor, which, as Pfliiger thought, 

 determined the building up of the embryo : but he overlooked the 

 fact that isotropism can only be referred to the body of the egg, 

 and that besides this cell-body there is also a nucleus present, from 

 which it was at least possible that regulative influences might 

 emanate. Upon this point Born J first showed that the position of 

 the nucleus is changed in eggs which are thus placed in unnatural 

 conditions, and he proved that the nucleus must contain a principle 

 which in the first place directs the formation of the embryo. Roux 2 

 further showed that, even when the effect of gravity is compensated, 

 the development is continued unchanged, and he therefore concluded 

 that the fertilized egg contains within itself all the forces necessary 

 for normal development. Finally, O. Hertwig 3 proved fi#m observa- 

 tions on the eggs of sea-urchins, that at any rate in these animals, 

 gravity has no directive influence upon segmentation, but that the 

 position of the first nuclear spindle decides the direction which will 

 be taken by the first divisional plane of segmentation. These 

 observations were however still insufficient to prove that fertiliza- 

 tion is nothing more than the fusion of nuclei 4 . 



A further and more important step was taken when E. van 

 Beneden 5 observed the process of fertilization in Ascaris megalo- 

 cephala. Like the investigations of Nussbaum a upon the same sub- 

 ject, published at a rather earlier date, van Beneden's observations 

 did not altogether- exclude the possibility of the participation of the 

 body of the sperm-cell in the real process of fertilization ; still the 

 fact that the nuclei of the egg-cell and the sperm-cell do not 



1 Born, 'Biologische Untersuchungen,' I, Arch. Mikr. Anat., Bd. XXIV. 



2 Roux, ' Beitrage zum Entwicklungsmechanismus des Embryo,' 1884. 



3 0. Hertwig, ' Welchen Einfluss iibt die Schwerkraft,' etc. Jena, 1884. 



4 [Our present knowledge of the development of vegetable ova (including the 

 position of the parts of the embryo) is also in favour of the view that it is not in- 

 fluenced by external causes, such as gravitation and light. It takes place in a 

 manner characteristic of the genus or species, and essentially depends on other causes 

 which are fixed by heredity, see Heinricher ' Beeinflusst das Licht die Organanlage 

 am Farnembryo ? ' in Mittheilungen aus dem Botanischen Institute zu Graz, II. 

 Jena, 1888. S. S.] 



5 E. van Beneden, ' Recherches snr la maturation de 1'oeuf,' etc., 1883. 



6 M. Nussbaum, 'Ueber die Veranderung der Geschlechtsprodukte bis zur Ei- 

 furchung,' Arch. Mikr. Anat., 1884. 



N 



