Papers from the Marine Biological Laboratory at Tortugas. 227 



(figs. 49 and 63). As the secondary bundles separate, the individual fibers 

 composing them fold back upon each other, so that several twists are 

 formed in the bundles (figs. 49 and 63). Owing to the shape of the spermato- 

 some, the secondary bundles reach the membrane in the anterior part of 

 the cell long before they do so in the posterior region. Correspondingly, the 

 differentiation of these structures to form the undulating membranes takes 

 place progressively from the anterior to the posterior end of the sperma to- 

 some. Consequently, a series of cross-sections passing in the opposite 

 direction through a spermatosome in the stage represented by figure 49 

 should show all the changes that take place both before and after the 

 secondary bundles reach the surface. 



Such a series is represented by figures 50 to 58. In figure 51 it is seen 

 that the secondary bundles, in moving apart, have broken the ring of 

 mitochondria which was mentioned before as having been formed around 

 the base of the primary bundle. Afterwards, the mitochondria come to 

 lie between the secondary bundles, while the dense cytoplasm which has 

 been associated with them is gradually dissolved (figs. 49 and 63). It will 

 be noticed that posteriorly the secondary bundles are wider than they are 

 anteriorly. As they approach the surface the fibers draw closer and closer 

 together; as a result, the bundles stain more and more deeply and it becomes 

 increasingly difficult to distinguish the individual fibers. It can be seen, 

 however, and very clearly too, that there is a modification taking place in 

 that those fibers which lie nearest the cell-membrane tend to draw slightly 

 away from the others and to fuse with one another (figs. 52 and 53). By the 

 time the bundles have reached the surface, the fusion of these outer fibers 

 has been completed and they are so closely applied to the cell-membrane 

 that the latter becomes indistinguishable from them (fig. 54). The next 

 figures show successive stages of the process by which the secondary bundles 

 push out the cell-membrane and come to extend laterally beyond the body 

 of the cell. Figure 58 represents a section passing just behind the centro- 

 somal plate and shows that the fibers have arranged themselves into definite 

 rows, a fact also indicated in figures 56 and 57. As will be shown presently, 

 this arrangement of the fibers is necessary for the complete formation of 

 the undulating membranes. 



At a somewhat later stage, after the secondary bundles have reached the 

 cell-membrane throughout their entire length, the myoneme-like striations 

 are formed. This process is shown in figures 59 and 60. Some of the 

 fibers which lie next to the cell-membrane now separate from their fellows 

 and begin to move out across the surface of the cell just beneath the mem- 

 brane. From this it would seem that there is not an actual fusion of the 

 exterior fibers of the secondary bundles, but that they lie in very close 

 contact with each other. It is probable, too, that the exterior fibers do not 

 participate in the twisting of the secondary bundles spoken of above. 

 Since the fibers are attached at both ends, the twisting of the secondary 

 bundles must take place at their middle portions and doubtless proceeds in 



