19 and 18_b_.

21. Stage similar to 18_a_, one chromosome in double diamond form.

22. First maturation spindle in metaphase, chromosomes in single and
double diamond shapes.

23. Chromosome in single diamond or tetrad form, as they usually come
into the spindle.

24. Double diamond-form assumed before metakinesis.

25. The 26 chromosomes of an early metaphase.

26. First maturation spindle in metakinesis.

27. Equatorial plate of first maturation spindle in metakinesis.

28. Another spindle, showing three granules which are probably remains
of nucleoli.

29. Anaphase of first maturation mitosis, one centrosome divided.

30. Late anaphase.

31-32. Telophase, exceptional cases of division of the cell.

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, showi