1902.] NICHOLS — SPERMATOGENESIS ONISCUS ASELLUS LINN. 85 



year. The karyokinetic figures of the spermatogonia are always 

 scattered and it is impossible to distinguish between the early and 

 late ones. 



5'. The Maturation Divisions. 



In preparing for the first maturation division the meshes of the 

 nuclear network become coarser, the granules more distinct and 

 aggregated into separate threads, joined together by linin (Figs. 

 32-36). The manner of their origin again lends support to the 

 view concerning their individuality in the resting cell. A still 

 greater condensation of the granules leads to a shortening and 

 thickening of the chromosomes (Figs. 37 and 38), the final result 

 of which is the production of sixteen compact masses of chromatin, 

 still connected by linin threads (Fig. 44). Condensation does not 

 proceed at an equal rate in all the chromosomes of a nucleus. 

 Fig. 45<^ shows a small portion of a nucleus in which lie side by 

 side two chromosomes, in one of which the final dumbbell-shape is 

 almost completed, while in the other the condensation of the chro- 

 matin is but little advanced. These sixteen masses are of various 

 forms. Some are dumbbell-shaped, two spheres of chromatin 

 joined by linin ; some are crescent-shaped and still others are more 

 or less complete rings (Figs. 39-45). The different forms may 

 occur in the same nucleus, but apparently without constancy in the 

 ratio of relative frequency of occurrence. The dumbbell-shape, 

 straight or slightly curved, is abundant, some cells containing no 

 complete rings (Fig. 41). Other cells contain a comparatively 

 large number of rings or crescents (Figs. 39 and 40). 



Two main types may be distinguished among the chromosomes 

 according to their structure and mode of origin — />., (i) those 

 in which the bivalent chromosome consists of two univalent chro- 

 mosomes lying end to end, as in those having the dumbbell-shape, 

 and (2) those in which the univalent chromosomes lie side by side, 

 as in those arising through a ring or narrow V-shape. A form in- 

 termediate between these is represented by those having a crescent- 

 shape. The different types and their probable mode of origin are 

 shown in the diagram (Fig. (>Zay d, c). It is interesting to note 

 that these types can be distinguished in the synapsis stage (Fig. 27), 

 although they are here not so well marked as in the prophases of 

 the first maturation division. 



In cells stained with iron haematoxylin, which have been strongly 



