THE SPERMATOZOON lOI 



the attraction-sphere of the mother-cell, from which the spermato- 

 zoon is formed in the testis. On the other hand, a few observers 

 have maintained, apparently on good evidence, that the centrosome 

 lies, not in the middle-piece, but at' the apex (p. 123). 



Reviewing these facts from a physiological point of view, we may 

 arrange the parts of the spermatozoon under two categories as 

 follows : — 



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



These are : — 



{a) The nucleus, which contains the chromatin and is to be 

 regarded as the vehicle of inheritance. 



{b) The centrosome, certainly contained in the middle-piece as 

 a rule, though perhaps lying in the tip in some cases. 

 This is the fertilizing element par excellence, in Boveri's 

 sense, since when introduced into the o,^^ it causes the 

 development of the amphiaster by which the &gg divides. 



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



viz. : — ■ 



(rt) The apex or spiir, by which the spermatozoon attaches itself 

 to the &g^ or bores its way into it. 



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

 centrosome, and, as it were, deposits them in the ^^^ 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. More- 

 over, as will appear beyond, it is nearly certain that the 

 contractile fibrillas are derived from the attraction-sphere 

 of the mother-cell, and therefore arise in the same manner 

 as the archoplasm-fibres of the mitotic figure — a conclu- 

 sion of especial interest in its relation to Van Beneden's 

 theory of mitosis (p. 70). 



Tailed spermatozoa conforming more or less nearly to the type 

 just described are with few exceptions found throughout the Metazoa 

 from the coelenterates 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. 48 and 74), conform very nearly to the 

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



