FORMATION OF CELLS. 



17 



FIG. 14.— Embryos in the embryo-sac oi Allium Cepa; the cells contain very 

 large nuclei, each with two nucleoli. At / the spherical apical cell contains two 

 nuclei {a) ; at // it has already divided (a has split up into a' and a"), and 

 same manner the cell c (in /) has split up into c and c'. 



ithe 



all the steps of the development in this case come easily into view ; here and there half- 

 formed partition-walls would be found. So is it also with the first cell-divisions of the 

 embryos in the embryo-sac ; here the circumstances are peculiarly favourable ; but here 

 also the next stage which 

 comes into view after the for- //^.^^"^^^^^ ^n 



mation of two nuclei "(Fig. 14, 

 7) is usually the presence of 

 a complete thin partition-wall 

 (//}. I was also successful in 

 crushing an embryo of Al- 

 lium Cepa {III) in iodine-solu- 

 tion in such a manner that it 

 was evident that the younger 

 derivative cells were not yet 

 separated by a partition-wall, 

 although sharply defined. 



b. While the diiusion of the 

 protoplasmic body is taking place 

 from Qvithout inwards, cell- 

 membrane is formed- a ridge 

 of cellulose intrudes into the 

 dividing fold which arises in 

 the protoplasmic body ^. 



A clear and well-studied 

 example is afforded in the 

 stouter forms of the genus Spi- 

 rogyra. In order to observe 

 the divisions here, it is neces- 

 sary to place strongly vege- 

 tating filaments after midnight 

 in very dilute alcohol, that 

 they may be examined later, 

 the divisions taking place 

 only by night. Fig. 15 shows 

 a living cell of a filament of 

 5. longata by day \ B to E the 

 conditions of division at night ; 

 the protoplasm - sacs of the 

 cells are contracted by the 

 life-destroying reagent. 



B and C (Fig. 15) show, at 

 q and q the folding-in of the 

 protoplasm - sac, and the an- 

 nular ridge of cellulose which 

 is growing into it. While the 

 folding - in advances further 

 and further, the lamella of cel- 

 lulose does the same; finally 

 the channel closes, the annular lamella 



Fig. xz.—Spn-o,i:yra loiitrata {X550). ^ a cell in the living state; B, C cells 

 laid in dilute alcohol during the division by night ; D, E central portion of 

 cells in the act of division. 



becomes a disc, and now lies between 

 two new completely closed sacs of protoplasm as a partition -wall. Sometimes 



the 

 the 



^ This case was the first of all processes of cell-foraiation that was accurately examined; H. von 

 Mohl first described it in 1835 in Conferva glomerata. (Mohl : Vermischte Schrifteu hot. Tnhalts. 

 Tubingen 1845.) 



