lOO 



THE GERM-CELLS 



entiated into a fin-like undulating membrane. The axial filament 

 may be traced through the middle-piece up to the head, at the base of 

 which it terminates in a minute body, single or double, known as 

 the cud-knob, and not improbably representing the centrosome. 



There is still some doubt regarding the nature and functions of 

 these various parts. The nucleus is proved both by its origin and 

 by its history during fertilization to be exactly equivalent to the 

 nucleus of the mature Q,^g. The middle-piece and the tail represent 





-f 



B 



a 



m 



-/ 



Fig. 48. —Spermatozoa of fishes and amphibia. [Ballowitz.] 



A. Sturgeon. B. Pike. C. D. I^euciscus. E. Triton (anterior part). F. Triton (posterior 

 part of flagellum). (7. TPrt/a (anterior part). ^. apical bofl\' ; tf. end-piece ; _/; flagellum ; /4. end- 

 knob (? centrosome) ; m. middle-piece; n. nucleus; s. apical spur. 



the principal mass of the cytoplasm of the sperm-cell, and the mid- 

 dle-piece is probably to be regarded as merely the thickened basal 

 portion of the flagellum. 



The principal uncertainty relates to the position of the centro- 

 some. It is certain that in most cases the centrosome or attraction- 

 sphere lies in the middle-piece ; for from it the centrosome arises 

 during the fertilization of the ^g^i ii"^ every accurately known case. 

 In a few cases, moreover, the middle-piece has been traced back to 



