34 Papers from the Marine Biological Laboratory at Tortugas. 



PLATE 2. 



FIG. 25. Very late telophase; mid-body prominent. 



FIG. 26. Resting-stage of the final order of secondary spermatogonia, showing a 

 chromatin nucleolus ("accessory chromosome"). 



FIGS. 27, 28, 29, 30, 31. Successive stages in the prophase of ensuing mitosis; acces- 

 sory chromosome in figure 30 distinctly bipartite. 



FIGS. 32, 33. Equatorial plates each with 35 chromosomes. 



FIG. 34. Two spindles at metaphase, showing accessory chromosome splitting at the 

 right of lower spindle. 



FIG. 35. Early anaphase of similar cell. 



FIG. 36. Later anaphase. 



FIG. 37. Telophase stage, showing two pairs of lagging chromosomes. 



FIGS. 38, 39, 40, 41, 42. Successive stages in the telophase of final secondary sperma- 

 togonial division. 



FIGS. 43, 44, 45. Still later steps in the telophase of similar division ; mid-body con- 

 spicuous in each spindle ; figure 44 shows persistence of accessory chromo- 

 some as a body of sharp contour and deep-staining capacity among the 

 pale-staining, irregularly shaped ordinary chromosomes. 



FIG. 46. Nucleus of primary spermatocyte in resting-stage. Reticulum delicate and 

 only very slightly chromatic. Accessory chromosome very close to nuclear 

 wall. This phase is of very brief duration, as also the following one. 



FIG. 47. Early growth-period of primary spermatocyte ; nuclear reticulum more highly 

 chromatic and with several karyosomes. 



FIG. 48. Late growth-period ; chromatin in form of close-meshed network of broad, 

 mossy, lightly-staining threads. Accessory chromosome closely attached to 

 nuclear wall. Amount of cytoplasm very small. 



FIGS. 49, 50, 51, 52. Successive stages in the presynaptic phase of the growth-period. 

 The accessory chromosome has become an elongate, club-shaped structure 

 attached at its narrower end to the presynaptic chromatin thread ; the 

 latter in the form of a close-meshed lattice-work. Figure 50 shows two 

 such cells still connected by cytoplasm of mother-cell. In figure 52 the 

 accessory chromosome is beginning to split longitudinally. 



PLATE 3. 



FIGS. 53, 54, 55, 56, 57, 58. Stages of synapsis. X = synaptic point. Accessory 

 chromosome is closely applied to nuclear wall ; frequently it is split as 

 in figure 57. Figures 57 and 58 may be identical with the " bouquet 

 stage " of Eisen. 



FIG. 59. Postsynaptic stage. Chromatin in form of an irregularly loosely-meshed 

 network of broad, mossy threads, which give indication of a longitudinal 

 split. The accessory chromosome is close to the wall ; the cytoplasm is 

 still scant. 



FIG. 60. Resting-stage of postsynaptic period (stained in methyl green). The acces- 

 sory chromosome has the appearance of a hollow sphere. 



FIG. 61. Resting-stage (stained in iron hematoxylin). Nuclear reticulum very delicate 

 and very slightly chromatic. Accessory chromosome solid and attached 

 to nuclear wall. 



FIG. 62. Resting-stage in which the reticulum is more chromatic; contains karyosomes 

 and has a bipartite accessory chromosome. 



FIG. 63. Early prophase of heterotypic mitosis; nuclear reticulum consists of coarser 

 and more highly chromatic threads ; accessory chromosome in form of 

 a deeply chromatic sphere some distance from nuclear wall. The sparse 

 cytoplasm contains several small masses of eliminated chromatin. 



FIG. 64. Later prophase; accessory chromosome clearly double. 



FIG. 65. Still later prophase ; pale-staining, mossy spireme partially segmented and with 

 indication of a longitudinal split. 



FIGS. 66, 67, 68, 69, 70, 71. Stages in the prophase of the heterotypic division. The 

 accessory chromosome (a) assumes various shapes, but always retains 

 ils definite contour and intense staining capacity. Several of the ordinary 

 chromosomes, which slain only slightly at this stage, are in the form 

 of tetrads. 



FIGS. 72, 73. Later prophases; all the chromosomes have similarly sharp contours and 

 intense staining capacity. Figure 73 has a ring-shaped tetrad and dumb- 

 bell-shaped accessory chromosomes (a). 



