136 



THE GERM-CELLS 



(attraction-sphere or "Nebenkern") of the mother-cell. It consists 

 of a fibrillated axial filament surrounded by a cytoplasmic envelope, 

 and in certain cases (Amphibia) bears on one side a fin-like undulat- 

 ing membrane (Fig. 66). Toward the tip of the flagellum the enve- 

 lope suddenly disappears or becomes very thin, leaving a short 

 end-piece which by some authors is considered to consist of the naked 

 axial filament. The axial filament may be traced through the 

 middle-piece up to the head, at the base of which it usually termi- 



Fig. 66. Spermatozoa of fishes and Amphibia. [BALLOWITZ.] 



A. Sturgeon. B. Pike. C. D. Leuciscus. E. Triton (anterior part). F. Triton (posterior 

 part of flagellum). G. Raja (anterior part), a. apical body; e. end-piece; f. flagellum; k. end- 

 knob; m. middle-piece ; n. nucleus; s. apical spur. 



nates in a minute body, single or double, known as the end-knob. 

 Recent research has proved that the axial filament grows out from 

 the spermatid-centrosome, the latter in some cases persisting as the 

 end-knob (insects, mollusks, mammals), in other cases apparently 

 enlarging to form the main body of the middle-piece (salamander). 

 The tail-envelopes, on the other hand, arise either from the "archo- 

 plasm " of the Nebenkern (insects) together with a small amount 

 of unmodified cytoplasm, or from the latter alone (salamander, rat). 



