298: INTRODUCTION TO CYTOLOGY 



tiation of some sort; so that the separation of the two halves of a trans- 

 versely divided chromosme would constitute a qualitative reduction. 

 If such actually is the condition of the chromatin, and if the chromosomes 

 do behave as Hutchinson supposes, a qualitative reduction must immedi- 

 ately follow each fertilization, and half of the resulting body cells must 

 have a constitution differing from that of the other half. Since there are 

 known no chromosome fusions in which a restoration in the number of 

 qualities is known to occur, the number of these qualities in a single 

 chromosome would in a few generations be reduced to one: in view of 

 the large number of past generations this must have already occurred. 



This new interpretation of chromosome behavior at fertilization and 

 the ensuing mitosis is thus seen to offer a direct challenge to those 

 theories of heredity that are based upon the idea of chromosomes carry- 

 ing linear series of differentiated units. It has now been put forward by 

 Hutchinson (1915) for Abies balsamea, by Chamberlain (1916) for 

 Stanger'ia paradoxa, and by Miss Weniger (1918) for Lilium philadel- 

 phicum and L. longiflorum. Consequently several investigators have 

 renewed the study of fertilization, and evidence contradictory to the 

 new theory has been found by Miss Nothnagel (1918) and Sax (1918), 

 whose researches are summarized in the following section on the 

 angiosperms. 



Angiosperms. The angiosperms are characterized by the occurrence 

 of "double fertilization," a phenomenon discovered independently by 

 Nawaschin (1898) and Guignard (1899). One of the two male nuclei 

 formed by the male gametophyte and brought into the embryo sac by 

 the pollen tube, enters the egg and fuses with its nucleus, thus forming 

 the primary nucleus of the embryo, while the other male nucleus fuses 

 with the two polar nuclei to form the primary endosperm nucleus (Fig. 

 123, B). As the male nuclei pass down the pollen tube they are usually 

 unaccompanied by any specially differentiated cytoplasm: the male 

 gametes are naked nuclei and not complete cells. In some cases, how- 

 ever, male cells have been reported (Fig. 123, A). When they are 

 liberated in the embryo sac by the rupture of the end of the pollen tube 

 any such cytoplasm is indistinguishable from that of the sac and that 

 discharged from the pollen tube. The male nuclei may appear in all 

 respects similar to other nuclei, or they may be distinctly vermiform, 

 as was observed by Mottier (1898) and later by many other workers 

 (Fig. 123, E). That such vermiform nuclei have the power of inde- 

 pendent movement has been held by Nawaschin (1899, 1900, 1909, 

 1910) for Lilium and Fritittaria, by Guignard (1900) for Tulipa, and by 

 Blackman and Welsford (1913) and Miss Welsford (1914) for Lilium 

 Martagon and L. auratum. The vermiform condition may persist until 

 the time of fusion, but in other cases, such as Fritillaria (Sax 1916), it 

 gives way to the ordinary shape. This change may occur more rapidly 



