52 EI SEN. [Vol. XVII. 



The mid-body is found on the central spindle at the end of 

 the anaphase and is, as has been often described, formed in a 

 vacuole between the two cell membranes. At the time when 

 the mid-body appears, the spindle bridge is always greatly con- 

 stricted in the middle and consists then of a lesser number of 

 fibers than during its earlier stages. The mid-body appears to 

 consist of a thickening or a concentration of the cytoplasm of 

 the fiber at one certain point. Thus there is either one thick 

 granule (Fig. 62) for each one of the thicker fibers, or there is 

 one granule connecting several of the fibers (Fig. 63). A solid 

 plate is never formed, and the individual granules of the mid- 

 body can always be recognized after proper differentiation. In 

 Fig. 62 we see how some of the fibers have become beaded 

 along their whole length, and we can easily understand that 

 the individual granules of the mid-body can consist of a con- 

 centration of the granules of the fibers. If this is a correct 

 explanation of the formation of the mid-body, then it is also 

 likely that the nature of the mid-body is the same as that of 

 the granular nodes of the individual fibers ; that is, a larger 

 storehouse for the cytoplasm of the fibers from which the fibers 

 may quickly draw a supply when, through the separation of the 

 cells, the spindle bridge is suddenly extended. The mid-body 

 would thus prevent the bursting of the spindle bridge at times 

 of unusual strain. Again when the spindle is compressed on 

 account of the pressure exerted by the retractile fibers, the 

 cytoplasm of the fibers can be quickly concentrated in the mid- 

 body, there to be stored for further use. This theory of the 

 mid-body is supported by the fact that in extended spindle 

 bridges (Figs. 13, 17, 32, 109) the mid-body is always absent, 

 evidently then having been used up by the extension of the 

 spindle bridge. 



V. Varieties of Cells. 



Polymorphous Spermatogonia. 



As polymorphous spermatogonia I designate the largest kind 

 of spermatogonia which, during their resting stage, possess 

 deeply folded or polymorphous nuclei. These polymorphous 



