Figure 18. Nucleus oi first spermatocyte of Lithohius sp. ? in early prophase. 

 Most of the chromosomes have left the karyosphere, and are in the form of 

 granular segments. 



TiGUKES 19a, 19b, 19", 20, 21. Later prophase of first spermatocyte in Lithohius 

 sp.l, showing various stages in formation of the tetrads and in disintegration of 

 the nucleoli. Figures 19*, 19'', and 19= represent successive sections of the 

 nucleus of one cell. 



Figure 22. Mid-prophase of first spermatocyte in Lithohius sp. ? The tetrads 

 have contracted to form denser bodies of chromatin. 



Figure 23. Nucleus of first spermatocyte, showing early stages in the forma- 

 tion of the tetrads in L. mordax. Some of the chromosomes have undergone 

 longitudinal splitting, resulting in double rows of globules. Others are already 

 taking on the form of characteristic tetrads. One chromosome is just arising 

 from the karyosphere. Tlie nucleolar material is already becoming vacuolated. 



Figures 24, 25. Mid-propliases of the first spermatocyte of L. mordax. The 

 centrosomes are moving apart along the inner face of the cell membrane. The fu- 

 sion of the globules forming each chromatid of the tetrad has begun. In two 

 chromosomes in Figure 25 this fusion is complete, there being only four parts to 

 the tetrad. The nucleolus is much reduced in size. 



PLATE 2. 



Figures 26, 27. Late prophase of first spermatocyte of L. midtidentatus. 

 Many of the tetrads have already become homogeneous. However, the dividing 

 planes are still very evident. The linin fibres are being converted into definite 

 threads. 



Figure 28. Late propliase of L. mordax. The nuclear membrane has dis- 

 appeared, and tlie linin fibres are becoming oriented to form the mantle fibres 

 of the spindle. 



Figures 29, 30. Division figures of the first spermatocyte of L. mordax, showing 

 the complete independence of nuclear spindle and astral systems. The fibres of 

 the spindle, however, converge toward the centrosomes. The distinction between 

 nuclear sap and cytoplasm is very distinct. The chromosomes are arranged in 

 an equatorial plate. At the point of attachment of the mantle fibres to the chromo- 

 some, a small, darkly stained body is seen. 



Figure 31. Cliromosomes seen in a single equatorial plate of L. mordax 

 sliowing the variety of shapes and sizes and the manner of attachment of the 

 mantle fibres. 



Figure 32. Mid-anapliase of fiirst spermatocyte of L. mordax. The daughter 

 chromosomes are of a distinct dumb-bell shape, the constriction representing the 

 plane of the second division. 



Figures 33, 34. Later stages in division of first spermatocyte of L. 7nordax. 

 Where chromosomes can still be distinguished, their form is unchanged. 



Figure 35. Late telophase of first, or early prophase of second, spermatocyte 

 of L. mordax. Cliromosomes still dumb-bell shaped. Centrosomes are moving 

 apart along inner surface of cell membranes. 



Figure 36. Metaphase of second spermatocyte of L. mordax. Achromatic 

 structures similar to corresponding stage of first division. The dumb-bell 

 shaped chromosomes are arranged with the constriction in the plane of the 

 equatorial plate. 



Figure 37. Telophase of second spermatocyte of L. multidentatus. 



