6s6 PATTERNS AND PROBLEMS OF DEVELOPMENT 



in early stages as particular embryonic regions or parts. Second, the fact 

 that embryos may develop not only from the egg but from other cells of 

 the embryo sac shows that the distinctive characteristics of the egg are 

 not essential to embryonic development and also suggests more or less 

 dedifferentiation in embryo formation from these other cells. Third, it is 

 evident from embryo formation by diploid cells of the nucellus, cells of 

 the gametophyte being haploid and the fertilized egg diploid, that embryo 

 formation is independent of these chromosomal differences. Fourth, the 

 data of polyembryony suggest that conditions in, or associated with, the 

 embryo sac are factors in determining the embryonic type of develop- 

 ment, whether from the egg or other cells of the gametophyte or sporo- 

 phyte, and that there is usually a differential in these conditions such that 

 the egg forms at the micropylar end of the sac; the more frequent develop- 

 ment of embryos from nucellar tissue about the micropylar region is also 

 suggestive in this connection. '^ 



There are also indications that the micropylar region of the gameto- 

 phyte is or becomes physiologically dominant, the degree or effectiveness 

 of this dominance differing in different species and probably under dif- 

 ferent conditions. In the light of the data on polyembryony, development 

 of a single embryo from the egg alone suggests a dominance effective in 

 inhibiting embryonic development of other cells. The fact that in many 

 cases of polyembryony the egg is not fertilized or, if fertilized, fails to 

 develop or ceases development at an early stage suggests that in these 

 cases dominance is largely lacking and that physiological isolation may 

 be a factor in embryonic development of other gametophyte cells than 

 the egg. If dominance of the gametophyte is slight or lacking, cells of the 

 nucellus, usually from the micropylar region, may attain dominance and 

 give rise to embryos. It is perhaps of some interest to note that the 

 spermatophyte embryo resembles a bud in certain respects. It develops 

 not from the whole egg but from one polar region of it. Is there any es- 

 sential difference between bud formation and development of the embryo 

 from the small active cells about one pole of the Gingko proembryo (Fig. 

 208, B)? According to this suggestion, the proembryo appears more nearly 

 comparable to the animal embryo, and the plant embryo represents the 

 first bud of the sporophyte, originating from what was primarily the high 

 end of the gradient system of the proembryo. 



And finally, in development of the gametophyte, position and polarity 



7 For more complete data on spermatophytes see Coulter and Chamberlain, 1903, 1910; 

 Coulter, Barnes, and Cowles, 1910; Goebel, 1922. 



