THE CELL DOCTRINE. 57 



admitted observations having reference to the cell 

 doctrine."* 



We have in the expression of this theory, a prac- 

 tical admission of the spontaneous origin of ani- 

 mal life, of which Dr. Bennett, in the paper re- 

 ferred to in the Popular Science Review, for Jan., 

 1869, openly declares himself the advocate. 



Closely allied to this theory is the so-called in- 

 vestment or cluster-theory (Umhullungs-theorie), de- 

 scribed by Virchow on page 53, of Cellular Pathology, 

 (Am. Ed. of Chance's Translation); according to which 

 " originally a number of elementary globules existed 

 scattered throughout a fluid, but that under certain 

 circumstances they gathered together, not in the 

 form of vesicular membranes, but so as to constitute 

 a compact heap, a globe (mass, cluster Klumpchen), 

 and that this globe was the starting-point of all fur- 

 ther development, a membrane being formed outside 

 and a nucleus inside, by the differentiation of the 

 mass, by apposition, or intussusception." 



FIG. 12. 



Fig. 12. Diagram of the Investment (cluster) theory. , Separate 

 elementary granules. A, Heap of granules (cluster), c, Granule cell, 

 with membrane and nucleus. 



TODD AND BOWMAN, 1856. 



Notwithstanding earlier approximations to the 



* Op. citat., p. 123. 



