52 ANATOMY AND PHYSIOLOGY OF 



fined intemalljj and to the interior of wMcTi tlie clilorophyll granules 

 adhere j he further as&ume& of the chloroph^^ll mass, that it is not sepa- 

 rated gradually from without inward, into two parts, but at once across 

 the whole cayity, and at this point the mass of mucilage at the same 

 time, and suddenly, forms a double layer as a cross wall, which secretenS 

 the true cell-membrane. These statements do not all agree with nature; 

 the formation of the septum is gradual, the time required for its forma- 

 tion amounts, according to Mitscherlich (" Monatsher. d. Ahad. zu BeTlin,'' 

 Nov, 1847) to 4 — 5 hours, 



Ohserv. 2. The division of cells without previous formation of a nticleus 

 appears to occur only in cellular plants and especially in the Algse. It 

 has been observed by ISTageli in Oscillatorise, Nostochinese and Diatomese. 

 It has been extended to the Phanerogamia by linger, who thought he 

 saw nuclei first appear in already formed cells in many cases, a state- 

 ment which certainly depends on error of observation. 



The division of the cells of the Desmidiese takes place in a man- 

 ner differing somewhat from the mode described in Conferva glo- 

 meTata, (See " Folke, Physiol, Studien!' 1 Heft ; Ralfss ''British 

 Besmidiem'' 5.) In these nnicelliilar Algse the cell consists of two 

 symmetrical halves, the boundary between which is sometimes 

 indicated only by a line {e, g.^ in Glosterium)^ and sometimes lies 

 hidden in an often very considerable constriction (e. g.^ Euastrum^ 

 GosmariuTYh). When the cell divides, these two halves of the cell 

 separate from each other, while a new portion is developed be- 

 tween them, consisting of a very delicate pellicle forming a con- 

 tinuation of the cell-membrane, and this new portion becoming 

 divided into two parts in the middle by a septum, the original 

 cell is separated into two, each of which is composed of half of 

 the original full-grown cell, and one of the very small rudiments of 

 a second half. This second half grows until it equals the older 

 half in size and shape, whereupon the subdivision begins again. 

 It is doubtful whether, as Ealfs assumes, the same process occurs 

 also in the division of the ceUs of the Nostochinese, Zygnemece 

 and many Oonfervm, 



In all cases of the division of cells in plants having a stem and 

 leaf, and likewise in many cases among the Thallophytes, the 

 formation of the septa is preceded by the development of as many 

 nuclei as there are subdivisions formed in the cell. The mode of 

 origin of these nuclei is two-fold ; either they are formed anew, 

 or an existing nucleus separates, by division, into several. 



When nuclei are formed anew in a cell, masses of protoplasm, 

 not sharply defined outside and increasing in density inwards, be- 

 come accumulated at the points where the nuclei are to appear. 

 Later on, especially by treating with iodine, we may observe in 

 the middle of each of these masses a globular body formed of 

 mucilaginous granular substance, more homogenous and frequently 

 far more transparent than the surrounding protoplasm, often 

 clearly defined externally, and almost without exception contain- 



