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 arc 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 T |j to j 1 ^ 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 lower 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 



