A Cytological Study of the Semi-parasitic Copepod, Hersilia apodiformis etc. 41 5 



of this sort is seen in Figure 21 (Plate Vj, to the riglit of the 

 middle, partly covered by the strepsistene thread (r.), which lies trans- 

 versely in the nuclens. The Separation of the components and their 

 contraction is shown in Fignre E 2, 3 and 4, the latter being based on 

 such pre-tetrads as those in Figure 23 &, c. In the oöcyte, the further 

 condensation of the chromosomes (Figs. E 5—6) would result in a tetrad 

 with the "Querkerbe" nearer one end (the right, in this case) than the 

 other, when viewed in the evuatorial plate. Figures E 7—9 show the 

 production of the rod di-tetrad, the "Querkerbe" being nearer the ends 

 than the middle, as in Figure 25 g. Such an asymmetrical tetrad is fig- 

 ured by Agar ('12) for Lepidosiren (his Fig. 13 c, Plate XIII). 



Various other forms of tetrads are met with occasionally in the 

 niaturation spindles, and Figures F, G, and H show how they may arise. 

 If the Separation of the threads in the strepsinema begins at both ends 

 (Fig. 20, the transverse pair) and the chromosomes remain in contact 

 at or near the middle, tetrads of X-form result (Fig. 23, h—j). The pro- 

 duction of the type of tetrad shown in Figure 25 e from such an X-form 

 is represented in Figure H. The two sister chromosomes, in contact 

 with each other at the "Querkerbe", open out so that the two portions 

 of each come into contact lateraUy. The "Querkerlje" itself is not to be 

 Seen in such a tetrad, but it is certain from its form that whole chromo- 

 somes separate in the first niaturation division. This type of tetrad 

 reseml)les to some extent the butterfly tetrads of Nezara (Wilson '11) 

 and Notoneda (BROwisfE '13). There are also two types of cross-shaped 

 tetrads to be seen in the spermatocytes: those which stand upright in 

 the spindle (Fig. 25 c, d), and those which lie sidewise in the spindle 

 (Fig. 25, a, &). The former is diagrammatically pictured in Figure G; 

 the latter, in Figure F. The cross in Figure i*" is, I belle ve, merely a 

 modification of the four-parted ring tetrad; for in late prophases of the 

 first spermatocyte division, one often finds forms which are intermediate 

 between the ring and the cross form (Figs. F2, 25 a). By further conden- 

 sation a cross would result. The upright cross is more difficult to under- 

 stand, but a possible explanation is given through the form shown in 

 Figure 25 c. This is represented in Figure G. The rod-shaped tetrad 

 of the spermatocyte often shows at the middle a slight tendency toward 

 cross f ormation (Fig. 25, h) ; and if the sniall lateral arms should increase 

 in size, as in Syromastes (Gross '06), a cross would result showing a ver- 

 tical and transverse light line, the interchromosomal plane (Figs. G 2, 3). 

 It is most probable that the increase of the lateral arms would continue 

 only until the "Querkerben", at which the chromosomes tend to bend, 



Ärcliiv f. Zellforschung. XIII. 27 



