164 
WALTER P. THOMPSON 
10. Endosperm 
As soon as the contents of a pollen tube enter the embryo-sac all 
the nuclei within the sac except the egg nuclei begin to divide. The 
divisions take place simultaneously throughout the sac. The similar- 
ity of the stages in all the dividing nuclei is remarkable. Figure 43 
represents such a sac with all its nuclei in the same stage of division. 
The male nuclei can be seen entering at the left beneath the small 
second embryo-sac. Further down against the wall are two eggs. 
The divisions are repeated once or twice and with great rapidity. 
The nuclei which had all been large and loose in structure become 
greatly reduced in size and very dense as well as very numerous. 
In G. gnemon the subsequent events differ from those in other species 
and will be described separately. 
(a) G. gnemon. — In this species although the parietal layer of 
protoplasm becomes thicker and encroaches on the vacuole, never- 
theless the latter remains for a long time. In the thick band of pro- 
toplasm one finds a large number of small, deeply staining nuclei. 
The number is particularly large in the mass of protoplasm at the 
bottom of the sac and in that surrounding the egg. Then at the base 
of the sac walls appear in such a way as to form compartments. Each 
of the latter contains a mass of protoplasm and several (up to 10) 
nuclei. The walls are formed first at the extreme base of the sac and 
then gradually higher and higher up in the parietal layer of proto- 
plasm. The result is a mass of cellular endosperm in the form of a 
shallow cup at the bottom and the parietal band of free-nucleated 
protoplasm above. In each cell are several nuclei. Figure 55 repre- 
sents a sac in which walls are just beginning to form at the bottom, 
and figure 56, a later stage in which walls are forming higher up in the 
parietal protoplasm. It will be observed that each compartment 
is multinucleate. 
The next step is the fusion of all the nuclei in each compartment 
into a single mass. The fusion takes place first in the lowermost 
compartments and then progressively in the higher ones. Conse- 
quently in certain sacs various stages in the process can be observed. 
Figure 57 represents a section of the base of a sac in which fusion 
has taken place in the lowermost compartments but not in the highest. 
As this section is tangential the vacuole in the center of the sac does 
not appear. Figure 58 shows a later stage in which fusion has 
