64 THE CELL DOCTRINE: 



all further development, a membrane being formed 

 outside and a nucleus inside, by the differentiation 

 of the mass, by apposition, or intussusception." 



TODD AND BOWMAN, 1856. 



Notwithstanding earlier approximations to the 

 truth, we find free cell formation still admitted by the 

 eminent authorities, Toddand Bowman, as one mode 

 of origin of cells, so late as December, 1856, though 

 the spontaneous origin of organs is spoken of as ex- 

 ceedingly doubtful. After describing the elements 

 of the ovum, considered in its entirety as a nucleated 

 cell, and referring to the period after fecundation, it 

 is stated, " At this period the embryo consists of an 

 aggregate of cells, and its further growth takes place 

 by the development of new ones. This may be ac- 

 complished in two ways : first, by the development 

 of new cells within the old, through the subdivision 

 of the nucleus into two or more segments, and the 

 formation of a cell around each, which then becomes 

 the nucleus of a new cell, and may in its turn be the 

 parent of other nuclei ; and, secondly, by the forma- 

 tion of a granular deposit between the cells, in which 

 the development of the new cells takes place. The 

 granules cohere to each other in separate groups, here and 

 there, to form nuclei, and around each of these a delicate 

 membrane is formed, which is the cell membrane. The 

 nuclei have been named cytoblasts, because they appear 

 to form the cells ; and the granular deposit in which 

 these changes take place is called the cytoblastema. 



" In every part of the embryo the formation of 

 nuclei and of cells goes on in one or both of the ways 



