No. 2.] THE ECHINODERM SPERMATOZOON. 253 



the spermatid into the spermatozoon, these granules gradually 

 fuse into larger and larger bodies (Figs. 34, 35, 36, 39w), until 

 they come to form in the cytoplasm a small number {2-%) of 

 refringent spheres (Figs. 35, 36, 4gm): these finally fuse into a 

 single spherical mass, the mitosome (Figs. 41, 42, 4pn). The 

 mitosome apparently may at first take any position whatever in 

 the cytoplasm, but with the formation of the tail and the con- 

 sequent gradual diminution of the cytoplasm in the head of the 

 spermatozoon the mitosome soon becomes pressed upon by the 

 cell membrane, and is gradually drawn into the place of least 

 resistance, i.e., between the nucleus and the beginning of the 

 tail, its normal position in the mature spermatozoon. 



LaValette St. George's discovery in 1867 of this body, 

 named by him the " Nebenkern," introduced new questions into 

 cellular morphology and physiology. What is and whence 

 comes this body ? What purpose does it subserve .■* The dis- 

 coverer at first held that this "Nebenkern" is formed by the 

 condensation of a part of the protoplasm of the spermatid, i.e., 

 that it is of cytoplasmic origin (36). This view was adopted 

 by Metschnikoff, Balbiani, and Biitschli from results obtained 

 from their investigations upon various Arthropods, and by 

 Keferstein upon Molluscs. 



According to the more recent researches of LaValette St. 

 George, Platner, Prenant, and myself, the "Nebenkern" is 

 formed from the granules which are the remains of the nuclear 

 spindle, and which after the final mitotic division fuse into a 

 single large spherical mass. 



As to what part of the mature spermatozoon is formed by the 

 "Nebenkern," there seems to have been a great disagreement 

 among investigators, not only in the different animal groups, 

 but even in many cases with different species within the same 

 group. According to the view of Keferstein (33), LaValette 

 St. George (36), Metschnikoff (43), and Duval (27, 28), for 

 Molluscs, particularly the Pulmonates ; and of Grobben (30) 

 for the Decapods, the " Nebenkern " forms the head of the sper- 

 matozoon. According to the observations of LaValette St. 

 George (38), and Biitschli (25), upon Insects, and of Pictet (18) 

 and myself upon Echinoderms it forms the middle piece. 



