Spermatogenesis in spiders. 533 



tion of the stain has been carried on for a long time the planosome 

 shows very sharply against the densely-staining threads, and counts 

 have been made from such slides in order to determine the point in 

 question. In sectioning, these cells have been cut at all possible 

 levels so that it has been necessary to set some arbitrary standard 

 by which to judge cells suitable for counting. Only full sized cells 

 have been selected in which the acessory chromosome could be 

 seen. Such cells were usually viewed from a plane perpendicular 

 to the axis of the loop of the thread, (Fig. 51). It is obvious, that, 

 since the position of the planosome varies, we will not always 

 have it in the plane of vision even were it present in all of the 

 cells. This source of error is unavoidable, however, and it is 

 constant. 



One hundred and seventeen cells of the pachytene stage were 

 closely examined; of these ninety-seven showed the presence of the 

 planosome, and twenty cells failed to show this body. When we 

 take into account the errors to which such counts are subject and 

 further, that the error favors undercountiug, these observations make 

 it probable that all of the pachytene cells of the tufted variety 

 carry the planosomes. 



Another interesting point was brought out by these counts. Of 

 the ninetj^- seven cells which showed the planosome, fifty showed this 

 body lying on the opposite side of the nucleus from the accessory 

 chromosome. 



Prophase. The condition of the accessory chromosome is very 

 interesting in the prophase, as it always has the simple bivalent 

 form, such as seen in Fig. B. I have never been able to find any 

 indication that this body w^as made up of four elements. During the 

 early prophase we also find the planosome lying free among the 

 autosomes of the nuclear cavity (Fig. 53). 



After the chromosomes have lain scattered through the nuclear 

 cavity for a short time, the nuclear wall dissolves and they are 

 drawn into the spindle of the first division (Figs. 54 and 55). In 

 the cell shown in Fig. 54, the chromosomes and spindles appear wdth 

 unusual distinctness. 



The typical position of the accessory chromosome is shown in 

 Fig. 54. Here we see the simple two rod type which is the only 

 form found in the tufted variety. This is also seen in equatorial 

 view (Fig. 56). The behavior of the planosome is very important. 

 It lies free in the spindle unattached to any chromosome, as may 



