THE SPERMATOZOON 137 



From a physiological point of view we may arrange the parts of 

 the spermatozoon under two categories as follows : 



i . The essential structures which play a direct part in fertilization. 

 These are : 



(a) The nucleus, which contains the chromatin. 



(b) The middle-piece, which either contains a formed centrosome 



or pair of centrosomes (end-knob), or is itself a meta- 

 morphosed centrosome. This is probably to be regarded 

 as the fertilizing element par excellence, since there is reason 

 to believe that when introduced into the egg it gives the 

 stimulus to division. 

 2. The accessory structures, which play no direct part in fertilization, 



(a) The apex or spur, by which the spermatozoon attaches itself 



to the egg or bores its way into it, and which also serves 

 for the attachment of the spermatozoon to the nurse-cells 

 or supporting cells of the testis. 



(b) The tail, a locomotor 'organ which carries the nucleus and 



centrosome, and, as it were, deposits them in the egg at 

 the time of fertilization. There can be little doubt that 

 the substance of the flagellum is contractile, and that its 

 movements are of the same nature as those of ordinary 

 cilia. Ballowitz's discovery of its fibrillated structure is 

 therefore of great interest, as indicating its structural as 

 well as physiological similarity to a muscle-fibre. The 

 outgrowth of the axial filament from the centrosome is 

 probably comparable to the formation of spindle-fibres or 

 astral rays, a conclusion of especial interest in its relation 

 to Van Beneden's theory of mitosis (p. 100). 



Tailed spermatozoa conforming more or less nearly to the type 

 just described are with few exceptions found throughout the Metazoa 

 from the ccelenterates up to man ; but they show a most surprising 

 diversity in form and structure in different groups of animals, and 

 the homologies between the different forms have not yet been fully 

 determined. The simpler forms, for example, those of echinoderms 

 and some of the fishes (Figs. 66 and 100), conform very nearly to the 

 foregoing description. Every part of the spermatozoon may, how- 

 ever, vary more or less widely from it (Figs. 66-68). The head 

 (nucleus) may be spherical, lance-shaped, rod-shaped, spirally twisted, 

 hook-shaped, hood-shaped, or drawn out into a long filament ; and 

 it is often divided into an anterior and a posterior piece of different 

 staining-capacity, as is the case with many birds and mammals, 

 but it is probable that the anterior of these may represent the aero- 

 some. An interesting form of head is described by Wheeler ('97) in 



