ORIGINS OF EMBRYONIC PATTERNS 



655 



onic stages may give rise to whole embryos, some degree of dissociation 

 being apparently the initiating factor. The proembryo of Ephedra forms 

 by nuclear division without cell formation up to eight nuclei, after which 

 cell formation occurs, and each of the eight cells gives rise to an embryo 

 and suspensor. 



Among angiosperms numerous cases of polyembryony have been re- 

 corded. In some there is multiphcation of embryos by budding from the 



Fig. 211, .4 -C— Three stages of embryo and suspensor of a monocotyledonous angiosperm, 

 Sagittaria variabilis. A, B, early stages with primary axis oriented as in dicotyledonous forms, 

 one synergid below embryo at left; C, later stage, the single cotyledon from the terminal re- 

 gion, the stem tip from the lateral cells, 5 (after Schaffner, 1897). 



suspensor, but usually only one persists. In these cases all embryos de- 

 velop from cells of the zygote. In others a synergid may be fertilized and 

 give rise to an embryo or may develop without fertilization. Embryos 

 may develop from antipodal cells and apparently also from an endosperm 

 cell. They may also originate as buds from nucellar tissue outside the 

 embryo sac, more commonly from the region about the micropyle than 

 elsewhere. These cases of polyembryony present several points of inter- 

 est. First, they show that in at least some species the cells resulting from 

 division of the egg are not determined, or not irreversibly determined, 



