50 OUTLINES OF EVOLUTIONARY BIOLOGY 



of a wall parallel to the earlier divisions (Fig. 14, B, e). The 

 embryo now consists of eight cells or " octants." The next divi- 

 sions are parallel to its surface and cut off a superficial covering 

 of cells, known as the dermatogen (Fig. 14, C, d), from which 

 the whole of the epidermis of the adult shoot will be derived. 

 The mass of rapidly dividing cells within this soon becomes 

 differentiated into two parts, the plerome (Fig. 14, D,pr,pl), lying 

 in the axis of the embryo, and the periblem (Fig. 14, D, pe), lying 

 between the plerome and the dermatogen. The plerome will 

 give rise to the vascular system of the plant and the tissues asso- 

 ciated therewith, while the periblem will give rise to the cortical 

 tissues of the stem and root and the mesophyll or middle layer of 

 the leaves. The periblem of the root, and the root-cap, are really 

 formed from the uppermost cell of the suspensor (Fig. 14, C, li). 

 By further cell-multiplication and differentiation in the three 

 primary layers dermatogen, periblem and plerome all the 

 various tissues of the adult plant are produced. 



Thus we see that in the higher plants and animals alike the 

 development of the individual from the fertilized egg consists in 

 the first place of a process of cell-division, and in the second place 

 of differentiation between the cells thus produced, accompanied 

 by grouping of the differentiated cells to form the more or less 

 sharply defined tissues of the adult. 



