486 JEAN REDMAN OLIVER 



cap attains a maximum thickness of 0.8 n in front and gradually 

 thins away to its posterior edge, which is more or less clearly 

 visible in face view as a slightly curved line or band. Behind 

 this follows a lighter zone, ca., 1.5 m in width, succeeded in turn 

 by a darker band, which extends to the posterior end of the 

 head. 



Collum. The neck (fig. 33, n.) includes in front the anterior 

 centrosomes, and extends posteriorly as far as the anterior por- 

 tions of the posterior centrosome, but does not include them. 

 The noduli anteriores or anterior centrosomes are two in number 

 and are closely fused to the posterior wall of the head, and, lying 

 in slight depressions, often appear to be included within the 

 nucleus itself. In figure 38 a case is represented in which the 

 head had been detached from the neck, leaving the anterior cen- 

 trosomes with the latter. In a few instances there seemed to 

 be present a third anterior centrosome between the other two, 

 but in the great majority of cases two only could be made out. 

 ■The remaining portion of the neck is made up of the massa 

 intermedia, a homogeneous substance with no discernible struc- 

 ture. Through this substance stretch two delicate filaments unit- 

 ing the anterior centrosomes to the succeeding ones, while around 

 the whole neck a delicate membrane, a part of the original mem- 

 brane of the spermatid, forms the outer boundary. The attach- 

 ment of the neck to the head is usuall}^ asymmetrical, it being 

 displaced to one side of the median line (fig. 33). In other cases, 

 fewer in number, it may be exactly centered, or practically so 

 (fig. 37). 



Cauda. The tail is divided into three parts, the connecting 

 piece, the principal piece, and the terminal piece. 



Pars conjunctionis. The connecting piece (fig. 33, c.p.) includes 

 the proximal half of the posterior centrosome, and is limited 

 posteriorly by the distal half of the same, the annulus, or end 

 disc. The noduli posteriores are two in number, approximately 

 equal in size in the adult sperm, though a decided difference in 

 the earlier stages is the rule. From one of these the filum prin- 

 cipale, or axial filament extends throughout the whole length of 

 the tail. Surrounding this filament is a very compact sheath of 



