GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 169 



some. The envelope of the middle-piece is here formed out of the 

 general cytoplasm. 



In the mammals the recent work of Lenhossek on the rat ('98) and 

 Meves on the rat, guinea-pig, and man ('98, '99) gives a result agree- 

 ing in its broader features with the forms already considered. In all 

 these mammals the young spermatids are closely similar to those of 

 the salamander, containing two peripherally placed centrosomes, from 

 the outer one of which the axial filament grows out (Fig. 86). Meves 



Fig. 86. — Formation of the spermatozoon in mammals. [MEVES.] 

 A. Spermatid of man, showing centrosomes and axial filament. B. Spermatid of guinea-pig, 



with acrosome. C. Nearly mature spermatozoon, showing backward migration of the ring. 



D. Mature spermatozoon ; r. final position of the ring. 



a. Acrosome surrounded by cytoplasm of the cell-body, most of which is afterward thrown 



off; c. centrosomes ; c.p. connecting-piece ; f. flagellum ; k. neck, containing end-knobs ; 



s. remains of the sphere (idiozome). 



and Lenhossek differ somewhat in their accounts of the later history 

 of these centrosomes, though agreeing that both contribute to the 

 formation of the middle-piece. Lenhossek states that in the rat both 

 centrosomes persist at the base of the nucleus to form the end-knob, 

 which, as Jensen showed ('87), is double in this animal. Meves finds 

 the process to be more complicated, agreeing in the main with that 

 observed by him in the salamander. In man and the rat the inner 

 centrosome passes to the base of the nucleus and flattens against it 

 to form a small disc-shaped body. The posterior centrosome divides 



