8 FIRST GROUP. THALLOPHVTES. 



as the outer wall is free from them, the outside of the sphere appears clear and 

 transparent (Fig. 35, A] ; folds of the cell-wall run from the circumference to the 

 centre of each shield, and give it the appearance of being lobed in a radiate manner. 

 A cylindrical cell projects inward from the centre of the inner wall of each shield 

 almost as far as to the centre of the hollow sphere ; this is termed the handle or 

 manubrium. The flask-shaped cell (pedicel-cell) that bears the antheridium also 

 intrudes into it, thrusting itself between the four lower shields. At the central extremity 

 of each of the eight manubria is a roundish, hyaline cell, the head-cell (capitulum) ; 

 the frame-work of the antheridium is thus composed of twenty-five cells. Each head- 

 cell is surmounted by six smaller cells (secondary head-cells), and from each of 

 these proceed four long whip-like filaments, the numerous coils of which fill the interior 



FIG. 37. Nitella flexilis. A fertile branch of 

 natural size ; i internode ; * leaves. B upper part of a 

 fertile leaf b with the node A", and on it two lateral 

 leaflets nt> and two very young oogonia 5; a the 

 antheridium. C older leaf with two leaflets ; a mature 

 antheridium a, and two immature oogonia 5". />a half- 

 mature oogonium highly magnified. 



FIG. 36. t\'ilf/fajf(xi/is. A a nearly ripe antheridium at the end of the primary leaf, beside it two lateral leaflets ; i neutral zones ; 

 arrows show the direction of the ' streaming ' of the protoplasm. B a manubrium with its head-cell and the whip-like filaments, in 

 which the spermatozoids are formed. C extremity of a young filament. D middle portion of an older one. F. one still older. 

 F mature antheridial filament with spermatozoids G. C G magn. 550 times. 



of the antheridium (Fig. 36, B). Each of these filaments about 200 altogether is a row 

 of small disc-shaped cells (D, E, F), the number of which amounts to 100-200. In 

 each of these 20,000-40,000 cells a spermatozoid is formed, a thin, spirally twisted 

 thread, thicker at its hinder extremity, and with two long delicate cilia at the other and 

 pointed extremity (Fig. 36, G}. When the antheridium is ripe the eight shields 

 separate by the lessening of their spherical curvature, the spermatozoids escape from 

 their mother-cells and move about in the water ; the antheridia appear to break up 

 usually in the morning, and the spermatozoids continue in movement for some hours, 

 sometimes till the evening. 



