CYTOPLASMIC STRUCTURES IN THE SEMINAL EPITHELIUM OF THE OPOSSUM. 59 



and secondary spermatocytes, and young spermatids up to the stage represented 

 in figure 8. To be strictly accurate, I should add that below such cells as are repre- 

 sented in figures 10 and 1 1 one finds spermatocytes that have reached the end of 

 the growth period. The subsequent modifications of the head of the spermatid 

 take place while the spermatocytes are dividing. The next generation of sperma- 

 tids below the stage corresponding to figure 12 has progressed as far as the stage 

 represented in figure 8. 



THIRD PERIOD. 



At the end of this period the remnants of the idiozome have disappeared. The 

 spermatid then contains a number of small granules in one or several heaps (figures 

 15a and 156), whose size and staining properties suggest that they may be geneti- 

 cally related to these remnants. They are eliminated with the residual body. 



Concerning the centrioles, the migration of the ring is the characteristic feature 

 of this period. In figure 13 the ring is shown when the migration is half completed; 

 in figure 14 at the end of it. The process is certainly a rapid one, as both stages 

 are found side by side. From what has been said above as to the relationship 

 between the protoplasmic body of the spermatid and the middle piece, it follows 

 that the ring does not migrate as far as the posterior extremity of the protoplasmic 

 lobe (fig. 14). During this period, in preparations made after Benda's method, the 

 ring does not stain intensely, as do the other parts of the centriolar complex, and 

 finally can not be seen at all (fig. 15). Other changes in the centrioles are the 

 following: Instead of two granules connected by a thin filament (fig. 12), we find in 

 the next stage (fig. 13) three granules (a fourth one, which happens to be situated 

 just below the ring, is a mitochondrium) . A similar arrangement in Phalangista 

 was met with by von Korff, who thinks that the two anterior granules are fragments 

 of the proximal centriole, of which the posterior one moves gradually towards the 

 distal centriole and finally comes to lie quite near it. By analogy one might conclude 

 that a similar process has taken place here, although it was not actually observed. 



The large, pear-shaped granule becomes more elongated as the ring migrates, 

 as if the influence that carries away the ring had exerted itself on the former body 

 also. It remains visible until after the deposit of the chondriosomes on the axial 

 filament (fig. 14), but in the ripe spermatozoon it has disappeared (fig. 25). Inter- 

 mediate stages frequently show one large granule or two smaller ones, which stain 

 like centrioles and are found in the neighborhood of the head (figs. 16 and 24). To 

 me it seems probable that they proceed from the centriolar granule. In describing 

 a similar element in the guinea-pig Meves states that it gradually diminishes and 

 is finally reduced to a very small size. The centriolar process described by Oliver 

 breaks off from the anterior part of the distal centriole; it persists in the sperma- 

 tozoon and is connected with a corresponding fragment of the proximal centriole by 

 a thin filament. 



The sheath covering the axial filament in the region of the main piece increases 

 very much in thickness during this period. On the anterior extremity of the main 

 piece it appears as an annular swelling (figs. 13 and 14); towards the posterior end 

 it gradually becomes thinner. Within it the axial filament can usually be followed 



