58 CYTOPLASMIC STRUCTURES IN THE SEMINAL EPITHELIUM OF THE OPOSSUM. 



the close contact of the proximal centriole with the nucleus and later the location 

 of the centrioles between the branches of the nucleus; (3) the excrescence of the 

 distal centriole, which grows so large that it can cover the whole centriolar appara- 

 tus. So much, however, could be seen. Very soon it appears that the centrioles 

 are no longer in relation with the middle of the posterior side of the nucleus (von 

 Korff's "laterale Insertion," fig. B) ; then the distal centriole flattens out and finally 

 breaks in two, a distal ring and a proximal granule which carries the axial filament 

 (fig. c). Meanwhile, the centriolar excrescence described above undergoes con- 

 siderable growth and becomes a spherical granule close to the other centriolar frag- 

 ments (figs. B, c, and 10). When the head begins to assume its definite shape this 

 arge granule is found always located toward its branched extremity. At the end 

 of the second period that is, when the ring is about to begin its migration the 

 same body becomes elongated and pear-shaped and its thickest part shows a 

 differentiation in the form of a small, brilliant granule (fig. 12). At the same stage 

 the proximal centriole and the anterior fragment of the distal centriole are more 

 widely separated and connected by a thin filament. This arrangement is very 

 similar to that found at the same period in Phalangista and represented by von 

 Korff in figures 17a, 18a, and 19 of his paper, except for the presence of the large 

 granule. Something similar to this latter body is found in other mammals, for 

 instance in the guinea-pig. Meves describes in that species an excrescence of the 

 anterior fragment of the distal centriole which grows to a considerable size. Later 

 it breaks off, but is connected with one of the fragments of the proximal centriole 

 by a thin filament. Whether any such connection with any other part of the 

 centriolar complex exists in the opossum I have been unable to ascertain. A cen- 

 triolar process, formed by an anterior fragment of the distal centriole, but smaller 

 than in the opossum and appearing only after the migration of the ring, is described 

 by Oliver. 



The axial filament is at first uniformly thin (fig. 1 1 and earlier) . Toward the 

 end of this period a differentiation takes place, inasmuch as on the posterior part a 

 deposit of a somewhat lighter substance appears (fig. 12). Thus, the future main 

 piece can be distinguished from the middle piece. As in Phalangista, it is clear 

 from that time on that the protoplasm extends farther back than the middle piece, 

 a condition which differs from what is observed in other mammals, such as the 

 guinea-pig, rat, etc. 



The second period is characterized by the sudden appearance and disappear- 

 ance of the caudal tube. While very little can be said in this case about its differ- 

 entiation and regression, it should be stated that no indication was found of its 

 nuclear origin, a view which has found some supporters but which, in the presence 

 of Meves's observations and more recently those of Oliver, can be safely discarded. 

 Nor was there any indication of its participation in the constitution of the middle 

 piece, as Oliver supposes. In fact, the peculiar shape of the head and the great 

 width of the caudal tube make such a participation hardly imaginable. 



During this period of spermiogenesis the cells of the subjacent layer are found 

 to be first spermatocytes in the second phase of the growth period, dividing primary 



