ORIGINS OF EMBRYONIC PATTERNS 651 



difference. The parts of the Isoeies embryo are apparently determined in 

 relations not greatly different from those attained in Selaginella at the 

 stage of Figure 207, E.'' 



SPERMATOPHYTES 



The gametophyte of the seed plants does not become an independent 

 individual but is represented by cells of the embryo sac developing from 

 the megaspore within the ovule. Its axiate pattern coincides in direction 

 with that of the ovule; and the ovule originates much like a burl but be- 

 comes surrounded by one or more integumentary layers with an opening, 

 the micropyle, at the free end. Earlier stages of gametophyte develop- 

 ment usually consist of nuclear division without formation of cell walls 

 in the cytoplasm of the megaspore within the developing ovule, cell 

 walls, in so far as they occur, appearing later. 



Gametophytes of gymnosperms fcycads, conifers, etc.) consist finally 

 of many cells, and in most members of the group the eggs form within 

 archegonia; but there appears to be a progressively earlier individuation 

 of the egg in the evolution of the group, and in the most highly developed 

 forms archegonia do not appear. When archegonia are present, they are 

 usually at the micropylar end of the gametophyte with axes parallel to 

 the gametophyte axis, but in certain gymnosperms they appear elsewhere. 

 It has been suggested that their position is determined in reaction to the 

 position of the entering pollen tube.'^ In embryonic development the ear- 

 lier divisions of the zygote give rise to a proembryo. The "basal" region 

 of the proembr>^o, that is, the part toward the micropylar end of the 

 ovule or the neck of the archegonium, becomes the suspensor; and in some 

 forms other nonembr^'onic cells develop from it. The embryo develops 

 from the "apical" region of the proembr\'0 and stem tip, and cotyledons 

 from the apical region of the embryo. The suspensor usually elongates, 

 in some forms enormously (Fig. 208, A), and carries the embryo into the 

 nutritive endosperm of the gametophyte. But in Gingko it is massive and 

 many-celled; only the small-celled polar region is concerned in embr}^o 

 formation; and stem tip and cotyledons develop later from its apex (Fig. 

 208, B). In the embryo of Pinus (Fig. 208, A) only the small cells at the 

 tip of the suspensor form the embr\'0, and stem tip and cotyledons de- 

 velop from its apical region.'' An apicobasal gradient, indicated by dif- 



t For more complete data on embryonic development of bryophytes and pteridophytes see 

 G. M. Smith, 1938; Goebel, 1930; Campbell, 1918; Coulter, Barnes, and Cowles, 1910. 



5 Coulter and Chamberlain, 1910, pp. 420-21. 



' In Fig. 208 and the following figures relating to spermatophyte development orientation 

 is uniformly with embryonic pole upward. 



