HISTOLOGY AND CYTOLOGY 



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fused, and a narrow longitudinal slit between them can be made out. 

 This coming together of the chromatin threads just before the 

 formation of the chromosomes is termed synapsis, and it constitutes 

 a fundamental difference between this type of cell division and 



C H cT 



FIG. 124. Spermatogenesis. From Bourne. 



A , a spermatocyte preparing for the first or meiotic division ; a large karyosomc is present and 

 the rest of the chromatin is scattered in the form of granules on the achromatic network of the 

 nucleus ; on the top of the nucleus is the centrosphere with two centrpsomes. B, spireme with 

 a single row of chromatin granules ; the loops of the spireme show a distinct polarity. C, early 

 stage of synapsis, the chromatin granules and the spireme ribbon are divided longitudinally. D, 

 twelve bivalent chromosomes of various shape becoming arranged round the equator of the spindle. 

 E, metaphase of the meiotic division ; each bivalent chromosome is being divided into its two 

 components, one of which is being drawn to each pole of the spindle. F, nuclear telophase and first 

 division of the spermatocyte. G, two secondary spermatocytes preparing for division ; the 

 upper figure showing an earlier stage of mitosis than the lower ; the nucleus of each spermatocyte 

 contains twelve univalent longitudinally-split chromosomes. H, the upper spermatocyte shows 

 the metaphase, the lower the early anaphase of the first' post -meiotic (homceotype) division. /, the 

 two secondary spermatocytes have divided to form four spermatids ; the nuclei are shown in differ- 

 ent stages passing from the late anaphase to the resting condition. Note that the figures G, H, 

 J are drawn on a somewhat smaller scale than the rest. 



ordinary mitosis, and, as we shall see later, one that is of great 

 importance subsequently. Generally, as these double or bivalent 

 chromosomes move to take up their position on the equatorial plate, 

 each of them splits longitudinally and at right angles to the original 



2 B 



