l8o CONTINUITY OF THE GERM-PLASM AS THE [IV. 



development is continued unchanged, and he therefore con- 

 cluded that the fertilized egg contains within itself all the 

 forces necessary for normal development. Finally, O. Hertwig^ 

 proved from observations 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 fertilization is nothing 

 more than the fusion of nuclei-. 



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

 Beneden^ observed the process of fertilization in Ascaris mega- 

 locepJiala. Like the investigations of Nussbaum* upon the 

 same subject, 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 coalesce irregularly, but that their loops 

 are placed regularly opposite one another in pairs and thus 

 form one new necleus (the first segmentation nucleus), dis- 

 tinctly pointed to the conclusion that the nuclear substance is 

 the sole bearer of hereditary tendencies— that in fact fertiliza- 

 tion depends upon the coalescence of nuclei. Van Beneden 

 himself did not indeed arrive at these conclusions : he was 

 prepossessed with the idea that fertilization depends upon the 

 union of two sexually differentiated nuclei, or rather half-nuclei 

 — the male and female pronuclei. He considered that only in 

 this way could a single complete nucleus be formed, a nucleus 

 which must of course be hermaphrodite, and he believed that 

 the essential cause of further development lies in the fact that, 



' O. Hcrtwig, ' Welchen Einfluss iibt die Schwerkraft,' etc. Jena, 1884. 



^ [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 influenced 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 whicli are fixed by 

 heredity; see Hcinricher, ' Beeinflusst das Licht die Organanlage am 

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

 II. Jena, 1888.— S. S.] 



^ E. van Beneden. ' Recherches sur la maturation de Tocuf,' etc., 1883. 



* M. Xussbaum, ' Ueber die Vcriindcrung dcr Gcschlechtsprodukte bis 

 zur Eifurchung,' Arch. Mikr. Anat., 1884. 



