186 THE CELL-THEORY 



forties. 1 Criticism of the Schwann-Schleiden theory from 

 the zoological side was suggested by the study of the seg- 

 mentation of the ovum the developmental process in which 

 the multiplication of cells is most easily observed. The 

 segmentation of the ovum was well known to Schwann, for 

 the process had been described in the frog by Provost and 

 Dumas in i82/j.,' 2 in the frog and newt by Rusconi/' and an 

 elaborate study of the process in the frog had been made by 

 von Baer. 4 Schwann indeed suspected that there must be 

 some connection between the segmentation of the ovum and 

 the formation of cells, but he did not realise that the 

 cellular blastoderm of the chick was formed by the division 

 or segmentation of the egg-cell. 



Segmentation was soon found to be of widespread 

 occurrence. Von Siebold in 1837 described the process 

 in Entozoa, 5 and in the same year Loven saw segmen- 

 tation in CainfxTtnulana^ and Sars in the starfish and in 

 Nudibranchs. 7 



In 1838 Bischoff 8 observed segmentation in the 

 mammalian ovum, and the whole course of segmentation 

 in the ovum of the rabbit from the 2-celled to the morula 

 stage was carefully described and figured by Barry '' in 1839. 

 C. Vogt 10 in 1842 described segmentation in Coirgoniis 

 and Alytes. The discovery of segmentation in the ovum of 



1 Sachs, History of Botany, Book ii. 



- Ann. Sci. nat., i., pp. 110-14, 1824. Swatnmerdam is said to have 

 observed the 2-cellcd stage in the egg of the frog (Ribl. Nat., 1752), and 

 Rosel v. Rosenhof the same stage in the tree-frog (f/ist. tint, rananim 

 nostratium, 1758). 



*Dueloppementdelagrenouille commune, Milan, 1826. Biblioteca 

 italiana, Ixxix., 1836, and Muller's Archi-v, 1836. Agassiz is said by Vogt 

 (1842) to have seen segmentation in the Perch as early as 1831. 



1 Muller's Archiv, 1836. 



"' In Burdach, Die Physiologic ah Erfahrungswissenschaft, 2nd Ed., 

 vol. ii. 



' Wiegmann's Archiv, 1837. 



' Bcricht Vcrsamml. dcutsch. Naturf. in 1'rag, 1837. 



H Bcricht Versamm. dcutsch. Naturf. in Freiburg, 1838. Later in 

 his Entw. d. WirbcltJi., and in his papers on the development of the 

 rabbit. 



'' Phil. Trans., 1839. See particularly PI. vi., figs. 105-12. 

 '" Embryologie des Salmones 1842. 



