ORIGINS OF EMBRYONIC PATTERNS 



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pole, the three resulting cells being the egg and a synergid on each side of 

 it (Fig. 209, D). The fate of the nuclei at the opposite antipodal pole 

 varies in different families and species. They, or even the primary antip- 

 odal nucleus (Fig. 209, A), may degenerate; or more or less definite cell 

 walls may develop about them, and these cells may become very large 

 and undergo differentiation as nutritive organs or increase in number. 

 Axiate pattern of the embryo sac is primarily coincident in direction with 

 that of the ovule. 



Fig. 209, A-D. — Diagrammatic outline of angiosperm gametophyte development. A-C, 

 nuclei of first, second, and third divisions; D, egg between two synergids at lower pole in figure, 

 antipodal nuclei at upper pole and members of primary endosperm nucleus in middle. 



In development of the zygote the embryo is formed from the region of 

 the egg which protrudes into the embryo sac, the ''free" pole (upward in 

 Fig. 209, D), the suspensor from the region toward the wall of the sac. 

 Cotyledons and stem tip of dicotyledonous angiosperms develop from the 

 "free" pole of the embryo. Figure 210 shows the axiate pattern of a 

 dicotyledonous embryo and suspensor. The eight-cell stage of the embryo, 

 of which four cells are shown in Figure 210, B, consists of four terminal, 

 or apical, and four basal cells. From the four apical cells cotyledons and 

 stem tip develop. The plane in which the two cotyledons develop is ap- 

 parently also determined by factors external to the embryo. 



The primary axis of the monocotyledonous embryos is oriented like 



