86 CELL INTELLIGENCE THE CAUSE OF EVOLUTION 



Third The middle piece, or connecting piece, a large 

 cytoplasmic body lying behind the head and giving at- 

 tachment to the tail from which it is not always distinctly 

 marked off. At its front end it is in some forms (mam- 

 mals) separated from the nucleus by a short clear region, 

 the neck. 



Fourth The tail, or flagellum, in part at least, a cyto- 

 plasmic product, developed in connection with the cen- 

 trosome. From a physiological point of view we may ar- 

 range the parts of the spermatozoon under two categories 

 as follows : 



First 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 a pair of centro- 

 scmes (end-knob), or is itself a metamorphosed centro- 

 some. 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. 



Second The accessory structures, which play no direct 

 part in fertilization, viz ; (a) The apex, or spur, by 

 which the spermatozoon attaches itself to the egg or bores 

 its way into it. (b) The tail, a locomotor organ, which 

 carries the nucleus and centrosome, and as it were, de- 

 posits 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 indi- 

 cating its structural as well as physiological similarity 

 to a muscle fibre. 



"Tailed spermatozoa conforming more or less nearly 

 to the type just described are with few exceptions found 



