XX DEVELOPMENT OF GONADS 215 



becomes divided into eight octants (Fig. 46, d^). Each of 

 these then divides tangentially (i.e. parallel to the surface 

 of the sphere) into two cells (d^), the inner of which divides 

 again (d^), so that each octant is now composed of three cells. 

 Of these the outermost forms the shield, the middle the 

 handle, and the inner the head-cell : from tlie latter the 

 secondary head-cells and spermatic filaments are produced 

 by budding. The entire spermary appears to be a modified 

 leaflet. 



The ovary also arises as a single cell, but soon divides and 

 becomes differentiated into an axial row of three cells (Fig. 

 47, B^, ov, nd, stk) surrounded by five others {sp. c) which arise 

 as buds from the middle cell of the axial row {nd) and are 

 at first knob-like and upright (b^). The uppermost or distal 

 cell of the axial row becomes the ovum (b^, b*, ov), the 

 others the stalk {stk) and intermediate cells {nd, x) : the five 

 surrounding cells elongate, and as they do so acquire a spiral 

 twist which becomes closer and closer as growth proceeds 

 (compare b^ — b*, and Fig. 44, g, ovy). At the same time the 

 distal end of each develops two septa (b^) and, projecting 

 beyond the level of the ovum, forms with its fellows the 

 chimney or crown {cr) of the ovary. There is every reason 

 to believe that the entire ovary is a highly-modified shoot : 

 the stalk representing an internode, the cell nd a node, the 

 spiral cells leaves, and the ovum an apical cell. 



Thus while the ciliate Infusoria and Caulerpa furnish ex- 

 amples of cell-differentiation without cell-multiplication, and 

 Spirogyra of cell-multiplication without cell-differentiation, 

 Nitella is a simple example of an organism in which com- 

 plexity is obtained by the two processes going on hand in 

 hand. It is a solid aggregate, the constituent cells of which 

 are so arranged as to produce a well-defined external form, 



