14 
MORPHOLOGY OF THE CELL. 
and a rounded form. The next step is the absorption of the cellulose walls and the 
setting free of the young spores as primordial cells, which soon become clothed with 
a cell-wall. This cell-wall subsequently becomes differentiated in a manner which is 
described in detail on page 403. 
In other cases the wall of the mother-cell presents ingrowths at points correspond- 
ing to the lines along which the cellulose walls are formed, and this conveys the erroneous 
impression that the wall of the mother-cell grows inwards constricting the protoplasm 
and ultimately dividing it into four parts. A good example of this case is afforded by 
the development of the pollen-grains of Dicotyledons. Fig. 11, taken from Tropseolum 
minus, wiil serve to illustrate it. In a four nuclei are present, which have been formed 
in the manner described above with reference to Equisetum : they are connected by 
Fig. 10.— Mode of development of the spores of Egatsetum limosum (X 550) ; a group of four, b group of two mother-cells ; 
c and d mother-cells preparing for division, showing the nuclear disc ; e one with two nuclei ; f, g, and i division into four spores ; 
Ji abnormal formation of three spores from one mother-cell. 
fibrillse, and six cell- plates are present in the equatorial planes of the groups of fibrillae 
which connect the nuclei. The wall of the mother-cell is thickened, especially at two 
parts of its surface, but it presents no ingrowths ; but now {b) ingrowths begin to be 
formed at points which correspond to the superficial ends of the six cell-plates (/,^,^, 
Their growth does not, however, proceed so as to cause the constriction of the proto- 
plasm into four masses : it is soon arrested ; the cell-walls are formed simultaneously 
in the six cell-plates and are attached externally to the ingrowths of the cell-wall of 
the mother-cell. Later each mass of protoplasm forms a new proper wall around 
itself, which is the wall of the pollen-grain ; the thick cell-walls surrounding thern are 
absorbed, and the four pollen-grains are set free. (See also p. 551.) 
b. A cell-'^all is formed after each nuclear diuisicn] the resulting .cells may be arranged 
either tetrahedrally or in one plane. 
This mode of cell-division closely resembles that which takes place in a growing 
tissue. It may be very clearly observed in the formation of the pollen-grains of most 
Monocotyledons. Fig. 12 shows the process in Funkia o'vata. In 7 two nuclei are 
