PLATE XV. 



All drawings were made under the camera Incida, with an optical combination consisting of 

 a Zeiss 2 mm. apochroraatic objective and compensating ocular No. S. This produced, at the 

 drawing surface, a magnification of 1000 diameters. In the photomechanical reproduction of 

 the drawings this has been reduced to 750 diameters. 



Fig. 1.— A semi-diagrammatic view of a late spermatogonia! anaphase, showing the number, 

 position and arrangement of the chromosomes. 



Fig. 2.— Same as fig. 1. 



Fig. 3.— Telophase of the last spermatogonial division, showing the small size of the cell, 

 and an early stage in the loosening of the chromatic structures. 



Fig. 4.— Very early prophase of the spermatocyte, in which is shown the production of granu. 

 lar chromatin from the chromosomes of the spermatogonia. Portions of the disintegrating chro. 

 mosomes shown at a. At h appears one which does not lose its identity ; this is the accessory 

 chromosome. 



Fig. 5.— A somewhat later prophase, in which the chromatic thread is established. The ac- 

 cessory chromosome has become definitely specialized and has taken its characteristic peripheral 

 position. 



Fig. 6. — A still later prophase, in which the chromatin thread is shown very much contorted. 

 At this stage it evidently consists of many pieces. The accessory chromosome is viewed from 

 the side. 



Fig. 7. — A stage of the prophases in which the chromatin has become aggregated into a sin- 

 gle thread. The accessory chromosome is here exhibited in cross-section. 



Fig. 8.— At this stage the single thread has become divided transversely into a number of 

 segments. Each of these, later, forms one of the elements which is found in the metaphase of 

 the primary spermatocyte. 



Fig. 9. — About the same stage as fig. 8, but showing the accessory chromosome. 



Fig. 10. — This represents a prophase stage closely following that shown in figs. 8 and 9. 

 Here, however, the chromatic segments have separated, taken a peripheral position, and become 

 finely granular. They have, as yet, assumed no particular form. 



Fig. 11.— In this the chro7iiatic segments are represented in their characteristic loop form. 

 The series of figures from 1 to 11 was drawn from cells in a single follicle, and, therefore, repre- 

 sents progressive development. The gradual and regular increase in size of the nucleus and 

 cell, from the stage represented in fig. 3 to that in fig. 9, is very noticeable. 



Fig. 12.— Here is represented the early longitudinal division of the chromatic segments. In 

 many cells it appears in a much more pronounced manner than is indicated. 



Fig. 13. — A cell at about the same stage as that regresented in fig. 11. This indicates the 

 great variation which the segments of the chromatic thread may undergo. 



Fig. 14. — Fragment of a cell, showing a portion of the nuclear elements. These are all seen 

 to be longitudinally divided. One of them is also bent at the center where the cross-division of 

 the second spermatocyte is to occur. The quadrivalent character of the element is thus clearly 

 indicated. By the subsequent concentration of the chromatic material these lines of division 

 become indistinguishable. 



Fig. 15. — The stage here represented follows closely upon that of the preceding figure. Two 

 of the elements yet show their quadripartite nature. The one in the center of the nucleus has 

 bent upon itself until the free ends have united to form a ring. 



Fig. 15a. — A somewliat later stage than fig. 15, showing a greater concentration of the chro- 

 matic elements. The longitudinal and cross-divisions are, however, still plainly visible. At 

 one side of the nucleus the astral rays are beginning to develop. 



Fig. 16.— A cell of the same age as that shown in fig. 15, but with the nuclear elements of a 

 more granular character and with no indication of division. 



Fig. 17. — This coll represents a stage a trifle in advance of ihat shown in figs. 15 and 16. The 

 nuclear element just below the accessory chromosome clearly presents the evidences of both 

 longitudinal and cross-division. It will be noticed that the chromatids are in contact along 

 the plane of cross-division for almost as great a portion of their length as they are along the 

 plane of the longitudinal division. This is a significant indication of future movements of the 

 elements. 



