REJUVENESCENCE IN NATURE. 237 



Spirogyra, that not the smallest doubt could remain of 

 the actual existence of the division completed by gradual 

 advance from the periphery to the centre.* The obser- 

 vation in Spirogyra is the more important, since a nucleus 

 exists in this genus, which assumes an equally important 

 part in the division to that it bears in the division of the 

 young cells of the Phanerogamia. Consequently this 

 takes away the principal reason brought forward by 

 Mohl in favour of the total difference of these two modes 

 of cell-formation, so that one might seek to explain the 

 distinction of the ordinary process of cell-formation, 

 characterised by the previous formation of new nuclei 

 and the sudden appearance of a line of fissure between 

 them, from the division by gradual constriction, by the 

 greater or less rapidity of an essentially identical process, 

 and to regard the first even as a gradually completed 

 division, only running through its course so rapidly that 

 the transitional stages are concealed from observation. 

 On the other side, it is true, an objection to such an 

 assumption may be found in the analogy of the process 

 of cell-division, such as occurs in young tissues, with_/ree 

 cell-formation. In both the new cell-formation is intro- 

 duced in the same way by the formation of the nucleus, 

 and if, in free cell-formation, a mass of contents situated 

 around the shortly previously formed nucleus, becomes 

 bounded and cut off simultaneously over the whole peri- 

 phery (so far at least as observation reaches), analogy 

 would lead us to suppose that in cell-formation where 

 the entire contents of the cell part into two masses, 

 surrounding the newly-formed nuclei, each of these 

 masses of contents will bring as much of its periphery 

 as is not otherwise previously determined, — therefore its 

 surface of contact with its fellow, — simultaneously to 

 definition and completion in its whole extent. The 

 sharpness of the distinction in the idea of ordinary cell- 

 division, according as we try to bring it into agreement 



* See the subsequent description. 



