SPERMATOPHYTA 



395 



Fig. 337. Successive stages in early development of the embryo of Capsella bursa-pastoris, 

 a dicotyledon, X500. A, two-celled proembryo; B, three-celled proembryo; C, proembryo 

 with longitudinally divided terminal cell and enlarged basal cell; D, terminal cell divided 

 to produce octants, four cells lying beneath the four shown; E, cutting off of dermatogen 

 by periclinal walls; F, differentiation of periblem and plerome, the former indicated by 

 lighter shading; the cell at the upper end of the suspensor is the hypophysis; G, completion 

 of periblem by cell cut off the hypophysis; H, later stage, showing cell divisions throughout 

 the embryo; 7, completion of dermatogen from middle tier of cells derived from the 

 hypophysis. 



transversely into two cells. The upper cell, called the hypophysis, con- 

 tributes to the embryo, while the lower one is added to the suspensor. 



The hypophysis divides transversely, the cell next to the embryo com- 

 pleting the periblem and the other cell undergoing tvv'o longitudinal divi- 

 sions at right angles to each other to form a plate of four cells (Fig. 

 337G, H). In a later stage, each of these four cells divides transversely, 



