EXTERNAL FACTORS 



III. i 



inside the egg towards the vegetative pole, and the distinction 

 between it and the protoplasm accentuated. The segmentation 



of such eggs is meroblastic ; a cap 

 of small cells is formed, a blasto- 

 derm, resting upon an undivided, 

 though nucleated, yolk, and these 

 yolk-nuclei are large and irregular, 

 resembling the giant nuclei of 

 the large-yolked eggs of Elasmo- 

 branchs and other forms (Fig. 46). 

 An experimental confirmation is 



thus afforded of Balf our's hypo- 

 ,,,<. i , 



thesis ; P ut forward on comparative 



grounds, that it is on the varying 



FIG. 46. Segmentation of the 

 Frog's egg under the influence 

 of a centrifugal force (from Kor- 

 schelt and H eider, after 0. Hert- 



wig). The egg consists of a bias- ,-, . n ,v *. -t-fe 



toderm and an undivided yolk quantity of yolk that differences 



(yolk-syncy tium) : kh, blastocoel ; in the segmentation of eggs pri- 

 m, yolk-nuclei ; d, yolk. 



If removed from the centrifuge in time, such eggs may con- 

 tinue to develop, though they frequently give rise to monstrosities 



(Spina bificla}. 



LITERATURE 



G. BORN. Ueber den Einfluss der Schwere auf das Froschei, Arch. 

 mikr. Anat. xxiv, 1885. 



0. HERTWIG. Welchen Einfluss iibt die Schwerkraft auf die Teilung 

 der Zellen ? Jen. Zeitechr. xviii, 1885. 



0. HERTWIG. Ueber einige am befruchteten Froschei durch Centri- 

 fugalkraft hervorgerufene Mechanoniorphosen, S.-B. konig. preuss. Akad. 

 Wiss. Berlin, 1897. 



L. KATHARINER. Ueber die bedingte Unabhangigkeit der Ent- 

 wicklung des polar differenzirten Eies von der Schwerkraft, Arcli. Eni. 

 Mech. xii, 1901. 



L. KATHARINER. Weitere Versuche uber die Selbstdifferenzirung dcs 

 Froscheies, Arch. Ent. Mech. xiv, 1902. 



F. EEIBEL. Bemerkungen zu Roux's Aufsatz ' Das Nichtnothigsein der 

 Schwerkraft fur die Entwicklung des Froscheies ', Anat. Am. xxi, 1902. 



T. H. MORGAN. The dispensability of gravity in the development of 

 the Toad's egg, Anat. Am. xxi, 1902. 



T. H. MORGAN. The dispensability of the constant action of gravity 

 and of a centrifugal force in the development of the Toad's egg, Anat. 

 Am. xxv, 1904. 



M. MOSZCOWSKI. Ueber den Einfluss der Schwerkraft auf die Entste- 

 hung und Erhaltung der bilateralen Symmetric des Froscheies, Arch. 

 mikr. Anat. Ix, 1902. 



