Origin and Development of the Vegetalle Embryo. 57 
through the canal of the outer coat, but it is very difficult to 
trace them (as Amici also remarks) through the very narrow canal 
of the inner coat. The pollen-tubes must not only become di- 
minished to a third or a fourth of their former diameter, but the 
refraction of the light in the cylindrical cells of the inner coat 
greatly interferes with distinct vision of its form. Some assist- 
ance is obtained by a very slight compression of the object, which 
is also necessary to expel air- bubbles which remain between the 
coats and in the canal of the inner coat, when the ovule is viewed 
in water ; and a microscope of the sharpest defining power is 
very desirable. A magnifying power of 200 diameters suffices, 
if its lenses be perfectly corrected. The lower end of the pollen- 
tube reaches the rounded apex of the embryo-sac, and turns 
toward the side to run a short distance sideways upon it. This 
of course can only be seen when a side view is obtained; if the 
pollen-tube lies above or below the embryo-sac, as the observer 
looks down upon it, he may easily imagine that it is in the interior 
of the embryo-sac. The circumstance that the pollen-tube follows 
the curved surface of the embryo-sac well supports the conclusion 
that it lies upon the outer side of the latter, and runs between 
its membrane and the inner coat of the ovule. The lower end 
of the pollen-tube swells up considerably in a clavate form, and 
then projects, especially at a somewhat later period, a good way 
into the embryo-sac, probably on account of the pressure it ex- 
periences from the coat of tlie ovule. The next phsenomenon is 
a change in the interior of the lower end of the pollen-tube and 
its inferior clavate expansion ; they no longer contain, like the 
upper part of the pollen-tube, a clear fluid in which granules 
are intermixed, and which has not the most distant resemblance 
to a tissue on the eve of development, or a protoplasm destined 
to the production of cells ; they now exhibit a coagulated, gru- 
mous mass, of a greenish-yellow colour. That this mass results 
from the transformation of the fluid contained in the pollen-tube 
is evident, from the fact that in certain cases the contents of that 
part of the pollen-tube outside the mouth of the ovule acquire a 
similar peculiarity. This coagulated condition of the contents of 
the lower end of the pollen-tube caused the author to feel doubt- 
ful at this stage of his inquiry as to the real point of origin of the 
embryo, since it seemed possible that this lower end of the pollen- 
tube was about to become developed into it. 
One of the three cells lying at the upper end of the embryo- 
sac now begins to grow ; in rare cases a second follows it in a 
similar development. The protoplasm of this cell is, as will be 
remembered, collected at the lower end ; in a short time a trans- 
verse septum is formed; a second and two more quickly fol- 
