168 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



duration, has been entirely missed in some plants, thereby creating 

 the false impression that the nucleus passes directly from the pro- 

 phase into the anaphase. Regarding the spindle, an intensity of 

 staining which is adequate or just right for the chromosomes often 

 fails to bring out the fibers, which are more clearly seen in over- 

 stained material. Finally, the division of the cell may take place 

 either by means of a constriction furrow or by the laying down of a 

 cell plate. 



Male "Cells" or "Nuclei." Formerly it was believed that when- 

 ever the division of the generative cell occurs in the pollen grain it 

 is followed by the formation of sperm cells, but if it takes place in 

 the streaming cytoplasm of the pollen tube only nuclei are formed. 

 Recent work has shown, however, that in all cases the male gametes 

 are definite cells and the cytoplasmic sheath persists throughout 

 their course in the pollen tube (see Schnarf, 1941). 



Considering first the case of Lilium, which has been the favorite 

 object for such studies, Guignard (1889) figured the male gametes of 

 L. martagon as cells, but Koernicke (1906), Strasburger (1908) and 

 Nawaschin (1910) believed that the cytoplasmic sheath is lost during 

 the division of the generative cell. As to the exact time of dis- 

 appearance of the sheath, however, these authors are not in agree- 

 ment with one another. According to Koernicke it is lost when 

 the generative cell is in prophase; according to Strasburger, at the 

 metaphase stage; and according to Nawaschin, only during the telo- 

 phase. Later, Welsford (1914) and O'Mara (1933) reported that 

 in L. martagon and L. auratum definite sperm cells are formed, al- 

 though eventually the cytoplasm dissolves away so that only the 

 naked nuclei enter the embryo sac. Cooper (1936) showed, how- 

 ever, that the male gametes persist as cells right up to the time they 

 enter the embryo sac. This has received further confirmation from 

 the work of Anderson (1939), who finds that the cytoplasmic sheath 

 around the male nuclei possesses all the inclusions normally present 

 in the vegetative cytoplasm. He explains that the failure of other 

 workers to see the sheath is due to their use of nuclear stains, which 

 are not suited for bringing out the cytoplasm to the best advantage. 



Not only in Lilium but also in other plants, the cytoplasmic sheath 

 around the male nuclei has been followed up to the time of their 

 discharge in the embryo sac. Nawaschin and Finn (1913) figured 

 a clear space around the male nuclei of Juglans, which, as Finn 

 (1925) subsequently explained, represents a thin layer of cytoplasm 



