148 



MORPHOLOGY OF FERTILIZATION 



long acrosome filament (Figs. 26b and 32). Typically, the reacted 

 spermatozoa show the middle piece rounded in outline, while the 

 filament and the base of the flagellum lie nearly at right angles 

 to each other. This appearance resembles that of reacted starfish 

 spermatozoa (Dan, 1954). 



Fig. 27. Reacted spermatozoa; bending and curving of acrosome fila- 

 ment, from sketches and photographs of hving specimens. (From Colwin 

 and Colwin, Biol. Bull., 1956.) a-h, Thxjone briareus; i-1, Asterias forbesii; 

 m, A. vulgaris (presumed distal part of acrosome filament dotted), a and 

 k-m, at egg surface; others, unassociated with egg. b, in egg water; c— h, 

 in alkaline sea water (d, same specimen as c, after separating from fixed 

 point); i-j, successive views of one specimen, in inseminated culture but 

 not associated with an egg. 



A striking feature of the acrosome filament of Thyone is its 

 great length. Whereas the longest acrosome filament previously 

 reported attained a maximum length of 22 to 28 microns ( in star- 

 fishes, Dan, 1954), the filament of Thyone occasionally is as 

 long as 75 microns (Fig. 29), and in one case it measured 90 

 microns. For comparison, the length of the flagellum is usually 



