MODE OF ORIGIN OF CELLS. 37 



may be observed under the . microscope in the fecundated egg 

 of the frog. 



No direct observations have been made as to the manner in 

 which in endogenous cell genesis the daughter cells are set 

 free in the body of the mother cell. 



In many cases we are acquainted with the stimuli through 

 the agency of which movements are occasioned. In the fecun- 

 dated egg the spermatozoa must be regarded as the agents 

 from which the first excitation proceeds. There can be no 

 doubt, also, that in the act of fission a high temperature plays 

 an important part (see above). In many other cases, however, 

 the stimulus inducing the fission of cells is unknown. 



The detachment of cells by constriction may be compared to 

 the act of birth. Before detachment occurs by this mode the 

 cell must have acquired a sufficient size, as otherwise material 

 limits would soon be placed to the process of continuous fission, 

 essential feature of cell multiplication, therefore, consists 



the capacity of assimilating new material. 



The general proposition, then, that cells may increase by 

 mstriction and detachment, cannot be called in question, 

 "he segmentation of the ovum is an example which admits of 

 LO double interpretation. It may nevertheless be disputed 

 whether certain cells of the adult organism are capable of 

 icreasing by constriction taking place in both the longitudinal 

 and the transverse direction. 



Since the migratory power of the white corpuscles has been 

 ascertained, some doubts may arise whether any other cells 

 besides these are capable of undergoing multiplication. With 

 the exception of cartilage, in which there can be no more doubt 

 of the occurrence of cell fission than in the fertilized egg, the 

 structures which result from the fission of cells in the tissues 

 of the healthy adult organization are not such as to render a 

 mistake impossible. In cartilage we see the descendants of a 

 parent cell enclosed in cavities of the solid matrix. But in all 

 other tissues, where such firm boundaries are not met with 

 surrounding families of cells, we cannot maintain with any 

 degree of certainty that a group of two or four cells lying in 

 close proximity to each other have originated in a previously 

 existing mother of equal physiological value; for it might 



