210 INTRODUCTION TO CYTOLOGY 



pollination^^ (Fig. 126). It has been shown in many cases^'' that the male 

 gametes are complete cells, at least while they are in the pollen grain or 

 tube, and sometimes until they reach the embryo sac (p. 236) ; but it is 

 also reported ^^ that their membranes sometimes disappear early, leaving 

 the male nuclei free in the cytoplasm of the pollen grain or tube. The 

 male nuclei in many species become vermiform, particularly after entering 

 the embryo sac'^ (Fig. 144). This change may occur more rapidly in one 

 nucleus than in the other, the two thus being unlike at late stages. It is 

 questionable whether or not such nuclei have any power of movement 

 aside from that occasioned by protoplasmic streaming and surface- 

 tension phenomena. 



In the ovule a megasporocyte is developed by the enlargement of a sub- 

 epidermal cell of the nucellus (Fig. 127). At the close of the first mitosis 

 a wall is formed between the nuclei, and the two resulting cells (secondary 

 megasporocytes) divide again, giving thus four megaspores. (In some 

 cases the outer secondary megasporocyte does not divide.) The outer 

 megaspores disintegrate, while the innermost one enlarges and develops 

 the female gametophyte. Its nucleus, which has the reduced number of 

 chromosomes, initiates a series of three divisions which result in eight 

 nuclei. Ordinarily four of these lie near each end of the elongated embryo 

 sac." In the micropylar end membranes are differentiated in the cyto- 

 plasm about three of the nuclei, forming an egg and two synergids, 

 while in the opposite end three antipodal cells are similarly formed. 

 The two remaining free polar nuclei move together and fuse sooner or 

 later. The angiosperm egg usually shows very little special morphological 

 differentiation, and except for its size and the position of its nucleus and 



1^ This constitutes a systematic character of some value (Schiirhoff, 1926, p. 260); 

 see this work for a review of the cytology of angiosperms. W6ycicki (19266) attrib- 

 utes the movement of the generative cell into the pollen tube to the action of vacuoles. 



i« E.g., Ishikawa (1918) on (Enothera; Wyhe (1923) on Vallisneria; Piech (19246, 

 1928) and Kostrioukoff (1930) on Scirpus; Trankowsky (19306) on Hemerocallis; 

 Madge (1929) on Viola; Finn (1925, 1926a, 1928) on Asclepias, Vincetoxicum, and 

 Vinca; and Finn and Rudenko (1930) on Orohanche. 



^' E.g., Dahlgren (1916) on Primula; Trankowsky (1930) on Convallaria and 

 Galanthus; Schiirhoff (19196) on Sambucus; and Poddubnaja (1927) on Echinops. 



'^E.g., Mottier (1898a), S. Nawaschin (1899, 1900, 1909, 1910), Land (1900), 

 Guignard (1900), Blackman and Welsford (1913), Welsford (1914), Sax (1916, 1918), 

 and Heimans (1928). 



Fig. 127. — Megasporogenesis, development of female gametophyte, syngamy, and 

 early embryogeny in an angiosperm. 1-5, division of megasporocyte into linear quartet of 

 megaspores; meiosis occurs in these two mitoses. 6-12, development of female gameto- 

 phyte by innermost megaspore, this involving three mitoses. In 13 the largest cell in the 

 upper end of the embryo sac is the egg, and two polar nuclei are in the center. 13, discharge 

 of two male nuclei into embryo sac by pollen tube. 14, double fertilization, one male 

 nucleus fusing with egg nucleus and the other with polar nuclei. 15, embryo in two-cell 

 stage; endosperm in ccenocytic stage. 16, embryo has developed cotyledons, and endo- 

 sperm has become cellular. Semidiagrammatic. 



