124 



THE GERM-CELLS 



(Fig. 62, B). Benda describes the " Nebenkern " in the mammals as 

 consisting of two parts, one of which passes backward and takes 

 part in the formation of the tail-envelope, while the other passes 

 forward to form the apex (head-cap or apical knob) and represents 

 the attraction-sphere (archoplasm). A somewhat similar account 

 was given by Platner of the " Nebenkern " of pulmonates. Accord- 

 ing to the more recent work of Moore on elasmobranchs, both 

 middle-piece and apex are derived from the attraction-sphere, the 

 centrosome passing into the former (Fig. 62, A). 



The work of Platner and Field appears to have been carefully 



D 



Fig. 62. Formation of the spermatozoon from the spermatid. 



A. Late stage of spermatid of the shark Scyllium. [MOORE.] 



B. Spermatid of starfish Chcet aster. [FIELD.] 



C. Spermatid of butterfly Pygcera. D. Young spermatozoon of the same. [PLATNER.] 



a. apical body; a.f. axial filament; c. "centrosome;" e. envelope of tail ; m. middle-piece 

 (" small mitosoma" of Platner) ; n. nucleus; /. paranucleus (" Nebenkern," or " large mitosoma" 

 of Platner). 



done, yet there is good reason to believe that both these observers 

 are in error, since their results are contradicted by the history of the 

 spermatozoon in fertilization. As regards the insects, Henking's 

 observations on the fertilization of the butterfly Picris leave little 

 doubt that the sperm-centrosome is here derived from the middle- 

 piece; and, moreover, in the grasshopper Caloptenus, Wilcox ('95) has 

 traced the centrosome of the spermatid into the middle-piece. In 

 the case of echinoderms, Boveri, Mathews, and myself, confirmed by 

 several later observers, have independently traced the sperm-centro- 

 some to the middle-piece during fertilization, and have shown that 



