56 Mr. A. Henfrey on the Progress of Physiological Botany : 
yond its apex ; the border of its mouth is swollen into a kind of 
roll, and the canal leading from it to the nucleus has begun to 
diminish in diameter. The outer coat begins to elongate down- 
ward from the lower end of the ovule in the form of an obtuse 
hollow spur. The pollen-tubes have by this time reached the 
lower end of the placenta. 
About the end of the second week the embryo-sac has wholly 
displaced the outer cellular layer of the upper and larger half of 
the nucleus. How this occurs the author could not clearly 
make out, and he leaves undetermined whether these cells are 
compressed gradually until their cavities are obliterated, and 
whether their membrane finally becomes blended with the em- 
bryo-sac or is absorbed. The outer coat now projects beyond 
the inner, and the canal of the latter becomes sensibly narrower, 
the mouth of the outer coat still continuing widely open. The 
pollen-tubes form a dense interlacement of curling filaments 
with knot-like swellings upon the placenta; their diameter is 
from yyj to j-Q millim. 
The external form of the ovule remains henceforward without 
much alteration, but a series of changes of the highest import- 
ance now ensues in the contents of the embryo-sac. The mass 
of protoplasm collected at the upper end, which hitherto appeared 
in the form of a simple deposit in the interior of the wall of the 
upper part of the cell, begins to separate into three masses, 
rounded below, connected together above. These masses are the 
first traces of the formation of three contiguous cells ; the nucleoli 
of each of these cell- nuclei can be distinctly seen before any trace 
of their membrane is visible. No sharp line of demarcation be- 
tween the nuclei themselves, or between them and the proto- 
plasm, can originally be detected; this is either because the 
nucleus is subsequently formed by a firmer union of a portion of 
the protoplasm, or its substance differs so little from the sur- 
rounding protoplasm in optical qualities, that the line of division 
escapes the eye. The conversion of these masses of protoplasm 
into ovate cells, which become enlarged downward to reach about 
the middle of the embryo- sac, takes place rapidly ; the author 
states that he has reason to assume that this change takes place, 
as a rule, in twenty-four hours. In proportion as these cells be- 
come elongated downward, the protoplasm contained within them, 
enveloping their nuclei and originally occupying their entire 
cavity, is drawn downward toward the lowxr end ; that is, the end 
turned away from the apex of the nucleus. 
This is the epoch when the pollen-tubes, which proceed from 
the placentas in a very tortuous manner, enter the mouth of the 
ovule, and now. Prof. Von Mohl says, the more difficult part of 
the investigation begins. The pollen-tubes arc easily followed 
