60 CYTOPLASMIC STRUCTURES IN THE SEMINAL EPITHELIUM OF THE OPOSSUM. 



for a certain distance. The sheath exhibits a delicacy of structure, a transverse 

 striation, not represented in my drawings. This has already been described by von 

 Korff, but in my opinion is not so marked as his text-figure 4 would indicate. It 

 can be seen clearly in certain preparations only, and occasionally Ramon y Cajal's 

 method gives very good images of it. A similar structure has been observed also 

 by Retzius; not in Didelphys, however (1909, pages 125-126), but in the other 

 marsupials studied by him (1906). In one of these (Bettongia), instead of a trans- 

 verse striation there is a beautifully developed spiral. The existence of these con- 

 ditions is interesting, as it shows that such structures are not necessarily formed by 

 chondriosomes. I have also in mind, in this connection, the spirals, etc., described 

 by Retzius (1902, 1910) and by Koltzoff (1908); see also Duesberg, 1912, p. 687. 



In the region of the middle piece the axial filament is very little, if any, thicker 

 than at the end of the previous period (fig. 13). It should be recalled in this con- 

 nection that in the guinea-pig, during the second period, Meves found a small 

 vesicle in that region of the axial filament. The latter appears to be the direct pro- 

 longation of the walls of this vesicle; furthermore, it is thinner within the vesicle 

 than without. For these reasons Meves thinks that a thin sheath covers the axial 

 filament in the region of the middle piece and that this sheath is in direct continuity 

 with the much thicker sheath that covers the axial filament in the region of the 

 main piece. I found in the rat (1908) a similar arrangement and adopted Meves's 

 conclusions. Von Korff, however, does not seem to be thoroughly convinced of the 

 existence of this sheath on the middle piece, in Phalangista. He states (p. 252) : 



"Meves nimmt ausserdem noch eine dritte Hiille an, die dem Axenfaden direkt 

 aufsitzt; das Blilschen des Axenfadens, auf Grund dessen er ihre Existenz vermuthet, 

 habe ich nur einmal gesehen." 



In the opossum I have never been able to find this vesicle; furthermore, the 

 structure of the tail, as it appears in figure 13, makes it rather improbable that the 

 sheath of the main piece extends onto the middle piece. 



At the end of this period the major part of the protoplasm is eliminated. Here 

 again there is a close resemblance to the same process in Phalangista. While in 

 the guinea-pig (Meves, 1899) and in the rat (Duesberg, 1908) the protoplasm 

 accumulates on the side of the middle piece and is there eliminated by progressive 

 indentation, in the opossum (as well as in Phalangista) it flows to the anterior part 

 of the spermatozoon (figs. 15a, 19, 20, and 21). The cast-off masses (Regaud's 

 corps residuels) constitute for a time a continuous layer between the expelled sper- 

 matozoa and the next generation of spermatids (fig. 16). These masses contain, 

 besides the small granulations mentioned above, which are presumably remnants 

 of the idiozome, a number of larger bodies and vacuoles which will be described in 

 connection with the chondriosomes. The residual bodies are later taken up by the 

 Sertoli cells and undergo degeneration, the termination of which is very often a 

 transformation into fat. 



FOURTH PERIOD. 



Besides the changes that affect the chondriosomes, described in the next chap- 

 ter, two important modifications take place during this period: the rotation of the 



