1 8 Lawson . — The Gametophytes , Fertilization and 
layer of embryo-cells next to the suspensors first elongate and this is 
followed by the second, third, fourth, and so on in regular series. Several 
layers of the embryo-cells thus become very much elongated, and continue 
the function of the primary suspensors in carrying the tip of the embryo 
deeper down into the tissue of the endosperm. Fig. 44 shows an advanced 
stage in the development of the embryo. • It will be observed from this 
figure, that the elongation of these embryonal cells may exceed the length 
of the primary suspensors and that the first of them are the longest and 
appear gradually shorter as they approach the growing tip of the embryo. 
Summary and Conclusions. 
At the time of pollination the microspore consists of two cells — 
the tube-cell and the generative cell. Vestigial cells or nuclei representing 
the vegetative prothallial cells of the male gametophyte are entirely lacking. 
Pollination takes place late in March. Three or four pollen-grains 
become lodged on the top of the nucellus at the base of the micropyle, 
and while they enlarge considerably they show no further nuclear activity 
until the following spring. 
There are usually three or four pollen-tubes produced. At the time of 
the penetration of the tube into the nucellar tissue, the generative nucleus 
divides, giving rise to the body-cell and the stalk-nucleus. The tube thus 
contains one large cell and two free nuclei. 
It takes about ten days for the tip of the tube to reach the archegonial 
chamber. When this has been reached, the body-cell divides, giving rise 
to two sperm-nuclei. The two sperm-nuclei are of equal size, and no cell- 
wall is formed between them. There are therefore not two male cells 
formed, but simply two large nuclei which lie close together within the 
membrane of the body-cell. 
Only one megaspore germinates, which gives rise to the female pro- 
thallium. The megaspore first enlarges, and this is followed by free nuclear 
division. A central vacuole soon makes its appearance, and by increasing 
in size, forces the cytoplasm and free nuclei to the wall. During the growth 
of the vacuole, free nuclear division proceeds at a rapid rate. The presence 
of a megaspore-membrane could not be detected in the young prothallium, 
but during the later fertilization-stages an extremely thin membrane was 
observed. The prothallium very soon consists of a large central vacuole 
and a very thin parietal layer of cytoplasm in which the free nuclei are 
distributed at intervals. 
When the parietal layer has reached a certain thickness a series of 
walls are formed between the free nuclei, thus constituting the primary cells 
of the prothallium which are open and exposed to the sap of the vacuole on 
the inner side. By their inward growth the primary cells eventually close 
