33-36. Partial rest stage between first and second maturation divisions, two nucleoli present. Chromosomes in fig. 36 in form of double diamonds ready for metakinesis. 37-38. Second maturation spindle in metaphase. 39. Equatorial plate of second maturation spindle, 26 chromosomes. 40. Same in anaphase. 41. Four spermatid nuclei in one cell, each nucleus containing one nucleolus. 42. A later stage, showing elongation of nuclei, centrosome and sphere at posterior end. 43-45. Later stages in the development of the spermatozoa, nucleolus grows gradually smaller. [Illustration: STEVENS. PLATE I. N. M. S. del. TERMOPSIS ANGUSTICOLLIS.] PLATE II. _Stenopelmatus._ FIGS. 46-47. Nuclei of spermatogonia, showing 2 and 3 nucleoli (_n_). 48-49. Prophase of spermatogonial mitosis, showing two exceptionally large chromosomes of equal length. 50. Equatorial plate of spermatogonial mitosis, 46 chromosomes. 51-54. Spermatocytes in spireme stage, nucleus containing a nucleolus (_n_), and a chromatin element (_x_), which is attached to one end of spireme and gradually increases in size during growth stage of spermatocytes. 55. Spireme longitudinally split and showing the beginning of cross formation. 56. Spireme segmented, tetrads forming. 57. One split segment and a part of another connected by bands of linin. 58. More open cross and diamond forms; element _x_ conspicuous. 59-60. More contracted cross and diamond-shaped tetrads; linin bands shown in 60, where element _x_ is also present. 61. Different forms assumed by element _x_ during tetrad stage (figs. 56-60). 62-63. Diamond-shaped and contracted cross-shaped tetrads from metaphase of first maturation mitosis, showing linin connections. 64. Diamond-shaped tetrad with spindle-fibers attached; _a-a_, probably halves of one univalent chromosome; _b-b_, halves of the other. 65. Dyad from anaphase of first maturation mitosis. 66-67. Metaphase of first maturation spindle, showing element _x_ in different positions. 68. Late anaphase of same. 69-70. Equatorial plate of first maturation spindle, 23 chromosomes and element _x_ below, in fig. 69. 71. Chromatin massed at poles of spindle; element _x_ isolated at one pole. 72-73. Two resting spermatocytes of the second order, one containing element _x_, the other not. 74-76. Successive stages of breaking down of element _x_. 77. Prophase of second division; dyads evident, but no sign of _x_ in this or following stages. 78. Second spermatocyte division--metakinesis. 79. Same; late anaphase. [Illustration: STEVENS. PLATE II. N. M. S. del. STENOPELMATUS.] PLATE III. _Stenopelmatus._ FIG. 80. Telophase of second maturation mitosis. 81. Young spermatid, showing spindle-remains at _s_. 82. Spermatid showing a conspicuous chromatin element in nucleus, and spindle-remains (_s_) elongated. 83. Spermatid, showing centrosome (_c_) and divided spindle-remains (_s_ and _a_).