THE STRUCTURE OF CELLS 



greatly in number, especially in the equatorial zone. In these 

 fibres the primordium of the new cell-wall is laid down, in the first 

 instance as a viscous film, but which later, by the deposition of fresh 

 substances, becomes converted into the cellulose partition. Its 

 mode of formation is interesting because reasons have been shown 

 for supposing that some at least of the protoplasmic connections 

 between adjacent cells are primarily effected by the permanence of 

 such continuity through the membrane during its formation. 



FIG. 1-2. 



Diagram of the successive stages of a nuclear division. 

 A, spirem, with the fission of the chromatic linin. li, aster. 

 C, D, K, separation of daughter segments. F, reconstitution 

 of daughter nuclei. (After Flemming.) 



It may, however, happen that no membranes are formed in the 

 interzonal fibres, such as will serve to delimit the daughter cells 

 from each other. A complete series can be traced between the two 

 extremes. Thus in the first division of the spore-mother-cell of 

 Fegatella (Fig. 13), a cell plate is laid down, but not completed, and 

 it is not until after the next nuclear division that this wall (which 

 has shifted its position in the interval) becomes part of the final 

 partitioning membranes. Again, in the endosperm of seeds, some- 

 times the embryosac is transversely divided after the first karyo- 

 kinesis, but far more commonly a large number of nuclei are first 



