58 THE CELL DOCTRINE. 



truth, we find free cell formation still admitted by the 

 eminent authorities Todd and 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 deli- 

 cate 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 cytoblas- 

 tema. 



"In every part of the embryo the formation of 

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

 ways above mentioned, and, by and by, ulterior 

 changes take place, for the production of the ele- 

 mentary parts of the tissues."* 



* Todd and Bowman, The Physiological Anatomy and Physiol- 

 ogy of Man. Am. Edit., Philadelphia: 1857, p. 63. 



