SPERMATOGENESIS OF THE PIG. 21 



nucleus. From this disc, which is apparently the end-knob, a 

 cylindrical mass of material extends backwards uniting the two 

 centrosomes (Figs. 68 and 69). A tiny filament extending back- 

 ward from the middle of the posterior disc-shaped centrosome 

 makes its appearance and is undoubtedly a continuation of the 

 coarser filament which unites the two centrosomes (Fig. 69). 

 As the tail grows longer the connecting filament becomes thinner, 

 and the posterior centrosome, w r hich after division was disc- 

 shaped and had increased somewhat in size, becomes trans- 

 formed into a ring and it can be seen that the filament extends 

 directly through it and continues backward as the tail (Fig. 70). 

 The formation of the ring takes place simultaneously with the 

 rapid growth of the tail and one is led to believe that the perfora- 

 tion in the disc is partly due to the fact that the material formerly 

 occupying that space goes to help in building up the axial filament. 

 Shortly after the tail projects out of the cell the ring moves along 

 the filament, backward, and soon swerves over to one side in the 

 cytoplasmic mass (Figs. 71 and 72). Very frequently it can be 

 seen a considerable distance away from the axial filament long 

 before the latter is fully developed (Figs. 73, 74 and 75). This 

 seems to indicate that the posterior centrosome takes no further 

 part in the development of the filament and that the latter is 

 mainly developed from the anterior centrosome, no part of 

 which is discarded or thrown off. While most of the inner centro- 

 some passes into the formation of the axial filament a part of 

 it remains as the end-knob in the small middle-piece (Figs. 76 

 and 78), a condition similar to that found by McGregor in Am- 

 phiuma ('99). 



The posterior ring-shaped centrosome, after moving away from 

 the filament, sometimes divides (Fig. 73), but usually assumes a 

 spherical shape (Fig. 74) and an interesting point in connection 

 with this body is that during the final development of the 

 spermatozoan it is invariably thrown off with a big mass of cyto- 

 plasm (Figs. 74, 75 and 76). The casting off of a portion of the 

 cytoplasm during the last stages of the developing spermatozoan 

 has been described by Meves ('99) in the guinea-pig where it is 

 closely similar to the process which occurs in the spermatozoid- 

 formation in ferns; but the throwing off of a portion of the 



