STRUCTURE OF OVUM 175 



seen to develop, in exactly the same manner as in plants. 

 These nuclei had accordingly the same significance as the 

 nuclei of plants, and deserved the same name of cytoblasts 

 or cell-generators. The true nucleus of the cartilage cell 

 was probably in the same way the original generator of the 

 mother-cell. 



Having proved the identity in structure and function of 

 the cells of these selected tissues with the cells of plants, as 

 conceived by Schleiden, Schwann had still to show that the 

 generality of animal tissues consisted either in their adult or 

 in their embryonic state of similar cells. This demonstration 

 occupies the second and longest section of his book. 



His method is throughout genetic ; he seeks to show, not 

 so much that all animal tissues are actually in their finished 

 state composed of cells and modifications of cells, as that all 

 tissues, even the most complex, are developed from cells 

 analogous in structure and growth with the cells of plants. 



All animals develop from an ovum ; it was his first task 

 to discover whether the ovum was or was not a cell. It 

 happened that, some years before Schwann wrote, a good 

 deal of work had been done on the minute structure of the 

 ovum, particularly by Purkinje and von Baer. Purkinje in 

 1825 1 discovered and described in the unfertilised egg of the 

 fowl a small vesicle containing granular matter, which 

 he named the Keimblaschen or germinal vesicle. ' It dis- 

 appeared in the fertilised egg. As early as 1791 Poli had 

 seen the germinal vesicle in the eggs of molluscs, but the 

 first adequate account was given by Purkinje. In 1827 2 von 

 Baer discovered the true ova of mammals and cleared up a 

 point which had been a stumbling block ever since the days 

 of von Graaf, who had described as the ova the follicles now 

 bearing his name. 3 Even von Graaf had noticed that the 

 early uterine eggs were smaller than the supposed ovarian 

 eggs; Prevost and Dumas 4 had observed the presence 

 in the Graafian follicle of a minute spherical body, which, 

 however, they hesitated to call the ovum ; it was left to von 

 Baer to elucidate the structure of the follicle and to prove 



1 Symbolae ad ovi avium historiam. 



' 2 De ovi mammalium et ho minis genesi. 



3 De mulieruin organis, 1672. '* Ann. Set. nat., iii., p. 135, 1842. 



