] 68 THE PHENOMENON OF 



cell, in this case also the formation of the separating 

 membrane must be secondary, that is to say, the division 

 and opposite limitation of the contents of the two new 

 cells must take place before the septum can be formed 

 between them by a secretion derived from the two sides 

 and uniting at the surface of contact. Whether in this 

 kind of cell-formation, which is most simply distinguished 

 by the term cell-formation by division,* the separation of 

 the two cells takes place simultaneously over the whole 

 surface of contact, as Nageli seeks to demonstrate, or 

 advances gradually, as it were by an annular constriction, 

 from the periphery to the centre, as Mohl represents in 

 some examples, and linger assumes as the universal law 

 of vegetative cell-formation, is one of the most difficult 

 problems in the study of cells. In a subsequent passage 

 I shall endeavour to establish the existence of both con- 

 ditions. If, in the latter case, namely the division of 

 the contents by a gradually advancing incision and 

 shutting off, the secretion of cellulose kept pace with the 

 division, it would naturally result that the division would 

 seem to be effected by an annular process of the cell- 

 merabrane growing inward from the wall. 



If we have recognised that the secondary formation of 

 the cell-membrane is the general rule, there can only 

 exist one more difference of opinion, namely, as to when 

 the secretion of the cellulose or allied substance on the 

 surface of the contained mass commences. In most cases 

 the commencement of this secretion would certainly coin- 

 cide with the conclusion of the external limitation of the 

 cell-mass, as the moment in which the creation of the 

 cell, as an individualised sphere of formation, appears 

 completed, and the outward development of the cell 

 begins ; and the very young cell may therefore frequently 

 possess a delicate cell-membrane in such cases, where it 



* Unger, 'Ueb. merisraatisolie Zellbildung bei der Entwicklung des 

 Pollens,' denominates tliis process merismcUic cell-formation; (Nageli, 

 ' Zeitschr.,' 1844, p. 73,) calls it parietal cell-formation around the whole 

 contents. (Ray Society Trans., 1845, p. 253.) 



