SYNGAMY 239 



through an early free-nuclear stage, no walls appearing until some time 

 later (p. 173). In other cases walled cells may be formed from the start. ^^ 



Although the nuclei throughout the endosperm are all derived by 

 repeated division from the triple-fusion nucleus in the great majority of 

 plants studied, cases are known in which at least some of them may be 

 produced by the polar-fusion nucleus alone without the male, or by the 

 fusion product of the male and one polar, or by one polar alone. Com- 

 binations of these methods may be found in tjie same embryo sac. In 

 Petunia the polar-fusion nucleus divides before the pollen tube opens, 

 and four endosperm cells are formed. One male nucleus then fuses with 

 that in the micropylar endosperm cell. That portion of the endosperm 

 which is derived from this cell is triploid (21 chromosomes), while that 

 derived from the three other cells is diploid (14 chromosomes) (Ferguson, 

 1927). Cases of "mosaic endosperm" frequently reported may have their 

 explanation in such unusual modes of development. Probably more 

 often they are due to aberrant mitotic behavior on the part of endosperm 

 nuclei initially triploid. 



The propriety of extending the term "fertilization" to the endosperm 

 fusion may be questioned. Obviously this fusion is not a true syngamic 

 one. It is followed by a resumption of activity on the part of the nuclei 

 in the embryo sac; but since this may occur with no nuclear fusion, it 

 seems evident that it is due to some peculiar condition pervading the sac 

 after the pollen tube enters. Only rarely does the egg develop without a 

 nuclear fusion. An activating, or "fertilizing," effect can be recognized 

 in both fusions, but it now seems proper to regard the endosperm in 

 angiosperms as gametophytic tissue which, unlike that of gymnosperms, 

 is arrested in development until after the pollen tube enters the sac; it 

 then proceeds with development, usually with, but sometimes without, 

 the addition of extra nuclei. 



ANIMALS 



The story of syngamy in animals^^ is complicated by the fact that in 

 many cases the periods of "maturation" and fertilization may overlap. 

 In sea urchins the two meiotic divisions in the oocyte are completed before 

 the spermatozoon enters. In certain other cases maturation proceeds as 

 far as the metaphase of the first mitosis (certain annelids, nemertines, 

 mollusks, and insects) or the metaphase of the second mitosis (frog, 

 mouse) but goes no further unless penetration occurs. Finally, in Nereis 



*^See Coulter and Chamberlain (1903), Samuelsson (1913), Jacobsson-Stiasny 

 (1914), Dahlgren (1922, 1923), Stenar (1925), Svensson (1925), Schtirhoff (1926), and 

 Schnarf (1929). 



2^ In the preparation of this portion of the chapter the author has drawn freely 

 upon Prof. F. R. Lillie's Problems of Fertilization, (1919). See also Lillie and Just 

 (1924), Doncaster (1920), Agar (1920a), and Wilson (1925). 



