212 INTRODUCTION TO CYTOLOGY 



vacuoles it closely resembles the synergids; in fact, cases are known in 

 which synergids as well as eggs may develop into embryos. Commonly 

 the egg is somewhat larger than the other cells, has its nucleus in the end 

 toward the center of the sac, and has one or more large vacuoles. Visible 

 ergastic materials other than cell sap are usually not conspicuous, but 

 chondriosomes and plastid primordia are probably present regularly. 



Many variations of the process just described have been found in the 

 angiosperms.^^ Probably the commonest of these is that exemplified in 

 Lilium: here no walls are formed after either of the two mitoses in the 

 megasporocyte, the four nuclei lying free in the cytoplasm of the embryo 

 sac where they undergo one further mitosis to form a typical eight- 

 nucleate gametophyte (Sargant, 1896). The egg, therefore, is removed 

 from the product of meiosis by a single mitosis, instead of by three mitoses 

 as in the usual type of development. 



A condition intermediate between this and the usual one is seen in 

 Smilacina, where the walls form and then disappear, leaving the four 

 nuclei to divide to eight (McAllister, 1909). Of exceptional interest is the 

 situation in Plumbagella micrantha, described by Dahlgren (1915). Here 

 the four nuclei, formed free as in Lilium, divide no further : one of them 

 becomes directly the nucleus of the egg, two of them fuse like polar 

 nuclei, and the fourth disintegrates in the base of the sac. This is the 

 only known case among higher plants in which the gamete nucleus is the 

 immediate product of meiosis, as in animals. 



In several known instances the embryo sac is developed neither from a 

 single megaspore as in the usual type, nor directly from the primary 

 megasporocyte as in Lilium and Plumbagella, but from one of the second- 

 ary megasporocytes. In Cypripedium, for example, the nucleus of this 

 cell undergoes the second meiotic mitosis, after which another mitosis 

 occurs, giving four nuclei, one of which becomes the nucleus of the egg 

 (Pace, 1907). Embryo sacs with more than eight nuclei have also been 

 found. In Peperomia hispidula no walls are formed after the meiotic 

 mitoses, the four nuclei dividing twice in succession to give 16. An egg 

 and one synergid are organized, and the remaining 14 nuclei fuse (D. S. 

 Johnson, 1907). The attention of the geneticist may be called to the fact 

 that when all of the embryo-sac nuclei arise from one of the four products 

 of meiosis in a heterozygous plant, they may be assumed to be of the same 

 genetic constitution; but when they are derived from more than one, as in 

 several of the examples cited above, they may differ greatly, so that very 

 unusual correlations between the characters of the sporophyte and those 

 of the endosperm may be expected. In several instances two or more of 

 the described modes of development are known to occur in the same 

 family and even in the same individual. 



'° For the types of embryo sac development, see Rutgers (1923), Stenar (1925), 

 Schurhoff (1926), Chiarugi (1927), Schnarf (1929), and Modilewski (1929). 



