INTRODUCTION. 31 
form from the cushion-like thickening of the posterior part, or 
individual cells of the margin grow out, and repeat the mode 
of development exhibited by the spore-cell in the original germi- 
nation, producing new prothalli by a process of budding. These 
““proliferous” prothalli bear antheridia, but the pistillidia have not 
been observed upon them. 
At an early but variable period of the growth of the prothallus, a 
number of small cellular bodies, analogous to the pollen of flowering 
plants, are produced on its lower surface, and chiefly about its 
central thickened part. These, which are called antheridia, consist 
of globular cells enveloped by one or two annular cells, and con- 
taining a number of free cellules or vesicles or sperm-cells, within 
which a spiral fibre representing the spermatozoid, is coiled up. 
These spermatozoids consist of a flattened band, curled spirally in 
about three and a-half coils, and bearing along the outer edge cilia 
of considerable length. If these parts are placed in water, as they 
must be for the purpose of observation, the cilia are seen to vibrate 
with such rapidity, that when the spermatozoid is in active motion 
they appear only as a fringe of light. ‘The motion of these bodies 
consists of a rapid rotation around their axes, and this combined with 
their spiral form induces a forward movement of great velocity ; the 
movement is, however, without regularity, the spermatozoids darting 
here and there, or turning aside without rule. If they come in 
contact by the smaller end, with any fixed body, they often adhere 
by this point, and then revolve around their axes without advancing. 
By degrees the motion slackens, and the rotation is lost, a kind of 
vibratory motion only remaining, and this at length ceases; but the 
spermatozoids appear to undergo dissolution during this time, and 
when they come to rest often appear as shapeless masses. When 
the motion is artificially arrested, which is done instantaneously by 
applying iodine, the flattened band is seen to have a little rounded 
‘head, from which the coil runs back, increasing successively in 

