664 D. B. CASTEEL 



layer from the surface of the cell near which the nucleus lies. 

 See figures 12 and 15. Following these rearrangements, the 

 surface of the cell opposite the nuclear pole is indented and the 

 bounding walls of the invagination cavity thus produced (fig. 

 19) grow outward (fig. 23) to form a long tube which is desig- 

 nated the 'outer tube' of the developing spermatozoon (fig. 24 

 and following). Into this tube is carried the material which 

 formed the outer striated layer of the spermatocytes and sper- 

 matids. The striations now take the form of cilia-like processes 

 (although they are not mobile), which later dissolve and form 

 a gelatinous mass within the tube. However, a tuft of these 

 threads at the base of the tube persists for a longer period (figs. ■ 

 24, 25, 26, 27). At the opposite or nuclear pole of the cell a 

 finger-form process (f.p.) develops and the nucleus migrates 

 toward the base of this process, at the same time changing in 

 form (figs. 24 and 26). 



The next step in metamorphosis is the formation of the 'inner 

 tube' which develops within the outer tube (figs. 27, 28, 29 and 

 30) . The inner tube grows at the expense of the outer, for when 

 both are of nearly equal length, the length of the entire sperma- 

 tozoon is reduced by one-half. The nucleus migrates through 

 the gelatinous layer along the outer side of the outer tube (fig. 

 29) and eventually lies opposite the distal ends of both tubes 

 (fig. 30) . All of the mitochondria of the cell are drawn into the 

 distal end of the inner tube. The finger-form process is reduced 

 in size. 



While the spermatozoon is in the spermatophore sac within 

 the genital ducts of the female, the distal end of the outer tube 

 forces its way out of the inner tube (at a, fig. 30), and the outer 

 tube slips backward along the inner. With the completion of 

 this evagination process, the two tubes form one continuous 

 sperm-tube, one-half of which represents the inner tube and the 

 other half the original outer tube turned inside out (fig. 31). 

 The mitochondria now lie at one end of the spermatozoon, 

 within the original inner tube, while the nucleus, which has 

 retained contact with the original distal end of the outer tube 

 and has been carried backward with it, now lies within the end 



