364 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The very small granules visible in the nucleus before the disappearance of 

 the nuclear membrane, and which later become the fibers of the mitotic spindle, 

 have already been referred to, as well as the fact that after the division of the 

 spermatocyte into the spermatids these granules, which were the mitotic spindle 

 of the spermatogone and spermatocyte, are no longer included within the nuclear 

 membrane, but are scattered through the cytoplasm. With the beginning of these 

 changes which mark the transformation of the spermatid into the spermatozoon 

 these granules gradually fuse into larger and larger bodies until they come to 

 form in the cytoplasm a small number (from 6 to 8) of refringent spheres. 

 These finally fuse into a single spherical mass the mitosome. 



The mitosome apparently may at first take any position whatever in the 

 cytoplasm, but with the formation of the tail and the consequent 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 resist- 

 ance that is, between the nucleus and the beginning of the tail, its normal 

 position in the mature spermatozoon, where it forms the so-called " middle-piece.'' 



The tail is a round flagellum 0.2 [i to 0.3 p. in diameter, formed from the cyto- 

 plasm of the spermatid. Soon after the mitotic division of a spermatocyte into two 

 spermatids the cytoplasm of the spermatid begins to form a bulging which 

 increases into a large projection like an enormous blunt pseudopodium ; the cyto- 

 plasm continues to push or flow into this projection, and so it becomes elongated 

 and flask shaped, the body of the flask consisting of a large drop of cytoplasm 

 which is connected with the cytoplasm proper by a narrow neck. The continued 

 lengthening of the tail takes place with the elongation and diminution in the 

 diameter of the neck, together with a diminution in the size of the drop of cyto- 

 plasm at the tip, as well as by a diminution of the cytoplasm which has hitherto 

 remained around the nucleus and the mitosome in the cell proper. 



The tail then is the cytoplasm of the spermatid which has become modified 

 in a very special way. It is possible that its violent motions in the water are 

 expressions of some molecular change which the sea water brings about in the 

 protoplasm of the tail, for Field has noticed that spermatozoa when first removed 

 from the testis into sea water lie motionless. After a short time a slight motion 

 begins, which after a few minutes increases to the normal rapid motion. 



It seems most probable that the tail is in direct continuation with the cell 

 membrane which surrounds the spermatozoon. The cell membrane being morpho- 

 logically but the external slightly changed cytoplasm, probably differs very little, 

 morphologically not at all, from the tail, and these two the tail and the cell 

 membrane probably pass insensibly into one another. The mode of development 

 would seem to prove this. 



A delicate cell membrane surrounds the head of the spermatozoon, inclosing 

 the nucleus, centrosome, and mitosome. This membrane is best seen in cases where 

 from some mechanical cause a slight separation has taken place between the 

 nucleus and the mitosome, this membrane in that event being stretched but still 

 unbroken. It may also be seen stretching over the centrosome in cases where the 



