652 PATTERNS AND PROBLEMS OF DEVELOPMENT 



ference in cell size, rate of division, vacuolation, staining properties (Fig. 

 208, B), or some of these differentials, very commonly appears in the 

 proembryo or later, often developing gradually. Although the physiologi- 

 cal factors concerned in determination of these axiate patterns in gymno- 

 sperms are not known, the patterns constitute a definite, orderly series, 

 each new pattern originating in definite relation to a pre-existing pattern. 

 That this relation indicates a physiological relation can scarcely be 

 doubted. 



Fig. 208, A, 5.— Embryonic stages of gymnosperms. A, greatly elongated suspensor cells 

 of Pimis with embryo at tip (after Coulter and Chamberlain, Morphology of Gymnosperms 

 [1910]); B, proembryo of Ghigko (from Child, 1915c, reproduced from H. L. Lyon, 1904). 



Development of the typical megagametophyte of angiosperms is dia- 

 grammatically outlined in Figure 209. Following the first nuclear division 

 of the megaspore, the two nuclei come to lie near the two ends of the 

 young embryo sac (Fig. 2og,A); each of these divides again (B), and each 

 of the four once more, giving four nuclei at each end of the sac (C). One 

 nucleus from each group migrates toward the middle of the sac; the two 

 usually fuse sooner or later, forming the endosperm nucleus. More or less 

 definite cell boundaries develop about the three nuclei at the micropylar 



