STEUOTUBE AND DEVELOPMENT. 21 



PL II, figs. 9-16. The discoid nodal-cell is first divided 

 into two nearly equal portions (PI. II, f . 9) by a vertical 

 wall called the halving-wall (h). The two cells so 

 formed undergo further division by the growth of suc- 

 cessive walls as shown in PL II, figs. 10-13, the first cell 

 being cut off on one side of the halving- wall, the second 

 on the other, and so on, alternatively; and therefore 

 when numbered according to their order of origin the 

 odd numbers are on one side, the even on the other. 

 The first two cells are from the beginning larger than 

 the others, the succeeding ones are smaller in proportion 

 to their higher numbers, this relation becoming more 

 apparent as growth proceeds, and the first cell, styled 

 by De Bary the initial cell, more especially maintaining 

 its advance of the others. It is from the initial cell 

 that the sexual plant arises, the other cells of the 

 periphery developing as the rudimentary branchlets 

 forming the pro-embryonic whorl and decreasing in 

 size as they recede from the anterior side. The first 

 division of the initial cell which takes place is by " an 

 inclined tangential wall dividing it into two cells, an 

 inner one entirely within the node, and an outer one, 

 almost the whole of which projects beyond the node 

 and has an arched crown. The outer one then begins 

 to grow and form segments after the manner of the 

 apical cell of the stem" (PL II, figs. 15, 16). The 

 lowest segment is divided by a vertical radial wall into 

 two parts which grow out into two small processes 

 compared by De Bary to the stipulodes at the base 

 of the whorls of branchlets of the sexual plant (PL II, 

 figs. 11, 12). 



Though the pro-embryo was figured by several earlier 

 authors, its nature was not understood until Prings- 

 heim in 1862 elucidated its structure in his admirable 



