656 D. B. CASTEEL 



through a series of definite modifications at all times parallel 

 with certain developmental stages of the cell, argues strongly 

 against a parasitic relation. Moreover, although Spirochaeta 

 gallinarum is a common blood parasite of the fowl and its tick 

 in many parts of the world, it has never, to my knowledge, been 

 observed in the blood of chickens or in ticks living in the United 

 States. The evidence seems conclusive that the vesicular bodies 

 are component parts of the germ cells and are not of external 

 origin. 



FORMATION OF THE SPERMATOZOA 



The preliminary steps in the metamorphosis of the spermatid 

 have already been indicated. The stage shown in figure 23 is 

 produced by the invagination of that side of the cell .which lies 

 opposite the nucleus (fig. 19), and this invagination marks an 

 important phase in the series of transformations which eventually 

 produce the highly specialized spermatozoon of the tick. These 

 processes begin with the extension of the invagination cavity, 

 and soon this cavity with its surrounding walls is converted into 

 a hollow tube (o.t.) which rapidly extends outward from the 

 nuclear region of the cell until it reaches the relatively enormous 

 length shown in figure 27. This tubular outgrowth is designated 

 the 'outer tube.' In the early stages of its development the 

 cilia-like remains of the striated layer of the spermatid project 

 into the cavity of the tube, but these structures degenerate later 

 and form a gelatinous mass which partly fills the tube. The 

 shorter filaments which are grouped to form a tuft at the bottom 

 of the tube retain their individuality somewhat longer. 



While the outer tube is developing, certain changes are occur- 

 ring at the nuclear end of the elongating cell. The mitochondria 

 break up into yet finer granules which collect around the proximal 

 end of the tube (fig. 24). The cytoplasm opposite the base of 

 the tube now flows outward and somewhat to one side, at first 

 in the form of a blunt projection which becomes more extended 

 (fig. 25) and finally takes the form of a distinct finger-like process 

 (f.p.) as shown in figures 26, 27 and 28. During the period 

 represented by figures 23 to 27 the nucleus also undergoes exten- 



