140 FUNDAMENTALS OF CYTOLOGY 



spores then develops into the female gametophyte as the other three 

 degenerate. 



This process shows many variations in different genera of angiosperms, 

 although the general structure of the gametophyte is often essentially 

 the same after different modes of development. Typical development 

 occurs as follows. The meiotic divisions in the megasporocyte are 

 accompanied by cytokinesis, the result being a row of megaspores. The 

 innermost of these enlarges greatly as its nucleus initiates a series of three 

 mitoses, yielding eight nuclei lying in the common cytoplasm of the 

 embryo sac. Membranes are formed about six of the nuclei, forming a 

 group of three antipodal cells at one end of the sac, and another group of 

 three called the egg apparatus, consisting of an egg and two synergids, at 

 the end near the micropyle. In the cytoplasm of the sac lie the two polar 

 7iuclei; these are not to be confused with animal polar bodies, which are 

 immediate products of meiosis. 



Certain other types of development occasionally found may be men- 

 tioned. (1) The female gametophyte develops from one of the two cells 

 present after meiotic division I, two of the nuclei resulting from meiosis 

 thus being involved in the formation of an eight-nucleate sac (Allium 

 type) or a four-nucleate sac (Podosternon type). (2) A four-nucleate 

 gametophyte arises from a single megaspore {Oenothera type). (3) No 

 cytokinesis accompanies meiosis, all four nuclei dividing once to form an 

 eight-nucleate gametophyte {Adoxa type). (4) After no cytokinesis 

 the four nuclei divide twice, giving a sixteen-nucleate gametophyte 

 {Pcpcromia t^'pe). (5) After no cytokinesis at meiosis one of the four 

 nuclei remains near the micropylar end while the other three pass toward 

 the antipodal end of the sac. All then undergo another mitosis. In the 

 micropylar end this yields two monoploid nuclei as expected, but in 

 the antipodal end the three nuclei undergo fusion just as their division 

 begins, so that instead of six resulting nuclei there are only two, each 

 with three sets of chi'omosomes. The four nuclei in the sac, two mono- 

 ploid and two triploid, now undergo another mitosis, giving four mono- 

 ploid and foiu- triploid nuclei. When cell membranes have been formed, 

 the egg, synergids, and one polar nucleus are monoploid, while the other 

 polar nucleus and the three antipodal cells are triploid. This is the 

 Fritillaria type and occurs in several species of Lilium, long supposed 

 to develop like Adoxa. Since both meiosis and nuclear fusion may 

 result in qualitative differences among nuclei, the importance of these 

 phenomena to the geneticist working with such plants should be obvious. 



Syngamy. — A necessary preliminary to syngamy in seed plants is 

 pollination. The pollen is brought by insects, wind, or other agency to 

 the stigma. There it germinates by sending out a pollen tube through 

 one of the special germ pores in the exine. The wall of the tube itself 



