8 MARGARET C. FERGUSON 



divided and were separated by cross- walls, the lower four di- 

 vided again making three layers of four cells each, the middle 

 layer then elongated pushing the lowest cells down into the 

 endosperm. In Picea a fourth layer of cells was observed at 

 the base of the central cell. 



In 1872 Strasburger stated that the canal-cell loosened itself 

 from the &^^ and hung as a cap just beneath the neck- 

 cells, at the same time the egg-nucleus increased in 'size and 

 moved to the center of the corpusculum. He detected two cells 

 in the pollen-tube of several Gymnosperms, but considered that 

 such cells were extremely rare in the Ad ietmecs, as he had only 

 once found one in this group. The shrunken remains of these 

 cells were seen in the pollen-tube after fertilization. He be- 

 lieved that the pit of the pollen-tube remained closed, and that 

 the exchange-substance was apparently communicated by a 

 vacuole between the apex of the pollen-tube and the egg- 

 nucleus. After fertilization the central nucleus was dissolved, 

 and, in " abnormal " cases, four new nuclei appeared in the 

 central part of the egg, but there was strong evidence that 

 these did not develop into an embryo. Six years later (1878), 

 he observed one or more divisions in the pollen-grain shortly 

 before pollination. The small cells resulting from these divi- 

 sions were interpreted as rudimentary prothallium. Two large 

 primordial cells were demonstrated in the pollen-tube of Pimis 

 and Picca when the tube was just above the archegonium. Ac- 

 cording to Strasburger's interpretation at that time, the nucleus 

 in front was dissolved while the one behind entered the egg 

 and fused with its nucleus. This was a great advance on his 

 previous observations, but he still conceived of the pollen-tube 

 as remaining closed, and fancied that the protoplasmic contents 

 passed through the membrane directly while the starch was dis- 

 solved before its transmission into the ^^^' He was now con- 

 vinced that only a part of the contents of the pollen-tube was 

 taken up by the egg-nucleus, the remaining portion uniting di- 

 rectly with the egg-plasma ; but he was not certain whether the 

 protoplasm active in fertilization came in as a formless mass or 

 in the shape of a nucleus. 



Strasburger established the fact, in 1S79, '^^^^'^ ^^ ^^ ^^^ fore- 



