18 



HUMAN EMBEYOLOGY. 



Head 



Body 



Tail 



Head" 



NeckT 



BodyJ 



End piece 



FIG. 20. HUMAN SPERMATOZOA and its sheath as they pass from 



(after Retzius). the body into the uil 



A, Side view ; B, Front view. 





consists of prolongations of the axial filament and its 

 sheath, and it ends in the short thin end-piece. 



The Transformation of the Spermatid into the 

 Spermatozoon. As the transformation progresses the 

 nucleus of the 'Spermatid becomes the head of the 

 spermatozoon. The axial filament grows out from the 

 posterior centrosome of the spermatid, which divides 

 into two parts, one of which becomes the posterior Endpi ece| 

 centrosome of the neck of the spermatozoon, whilst the 

 other becomes the end-ring of the body of the sperrna- FlG . 2 i. -STRUCTURE OF A HUMAN 



tOZOOn. SPERMATOZOON (after Meeves). 



The anterior centrosome of the spermatid becomes 



the anterior centrosome of the 



.- Nucleus - - T^-L |\ Head nucleus. . /_ 



Anterior / \\^/ '\ Anterior centrosome^, 

 centrosom e ~ ~f - - ' ; :| 



Posterior centrosome _ -j 



Body-- 

 End ring 



The Body. The body is a little longer than the head, and its constituent parts 

 are : (1) a portion of the axial 

 filament ; (2) a portion of the 

 axial sheath ; (3) the spiral 

 sheath ; (4) the mitochondrial 

 sheath ; (5) the end-ring. 



The axial sheath is a thin 

 layer of protoplasm immediately 

 surrounding the axial filament. 



The spiral sheath consists of 

 a spiral fibril embedded in in- 

 different protoplasm, and the 

 mitochondrial sheath, which sur- 

 rounds the spiral sheath, is 

 formed by protoplasm contain- 

 ing numerous mitochondria. 



The end-ring closes the pos- 

 terior ends of the spiral and 

 mitochondrial sheaths, and it is 

 perforated by the axial filament 



Head cap 



Ant. centrosome 

 Post, centrosome 



Axial filament 



. Spiral sheath 

 Mitochondria! 



sheath 

 End ring 



Axial filament 



.Sheath of axial 

 filament 



Posterior ,,_| ^ 



centrosome 



Axial filament 



Tail 



can only be surmised. 



neck of the spermatozoon. The 

 cytoplasm of the spermatid forms 

 the axial sheath, the indifferent 

 protoplasm of the spiral sheath 

 and mitochondrial sheath. The 

 origin of the spiral filament and 

 the origin of the head-cap are 

 uncertain, but it is stated that, 

 in some animals, the head-cap is 

 formed from the accessory body, 

 which is not shown in Fig. 22. 



The Object of the Reduction 

 of the Chromosomes. The most 

 striking phenomenon of the pro- 

 cess of the maturation of the 

 gametes is the reduction of the 

 chromosomes. The constancy of 

 the reduction tends to emphasise 

 its importance, but, as we have 

 no definite knowledge of the 

 functions of the chromatic sub- 

 stance, the object of the reduction 

 The evidence which has been accumulated tends to the 



Tail 



Nucleus f 



Anterior 

 centrosome -_ 



Posterior 

 centrosome"" 



End ring- - X^ 

 Tail 



FIG. 22. SCHEMA OF TRANSFORMATION OF SPERMATID 

 INTO SPERMATOZOON (after Meeves, modified). 



