326 THOS. H. MONTGOMERY, 



Finally, it may be stated of the elements of the linin spirem, and 

 particularly of the distal threads, that sometimes they do not appear 

 to be thread-like in structure, but to be composed of rows of granules 

 (Figs. 161, 165); possibly this is due to variation in the fixation 

 method. A similar appearance of the linin was figured by me for 

 corresponding stages in Peniatoma (1898, figs. 152, 153, tab. 3). 



3. Monaster stage. 



The point has been reached in our description, when the nucleus 

 has assumed its greatest size, when the two pairs of centrosomes lie 

 at its opposite poles, and when the chromosomes have acquired their 

 definitive form but are still placed irregularly in the nuclear cavity 

 (Figs. 171 — 174, Plate 22). Near the end of the prophase most of 

 the chromosomes usually lie near the nuclear membrane ; this position, 

 like the peripheral position of the chromatin in cells generally during 

 the prophases, I would suggest might be due to the tension of those 

 delicate linin fibrils which attach the chromatin elements to the nuclear 

 membrane. 



A line joining the two centrosomes of a pair is not paratangential 

 to, but makes an angle with, the surface of the nucleus (Figs. 168, 169). 

 As was shown, each pair of centrosomes has moved along the nuclear 

 surface through an arc of 90" from its original position in the idio- 

 zome cup at the distal end of the cell. Accordingly, the axis of the 

 spindle now to be formed is perpendicular to a line joining the original 

 central and distal poles of the cell. The idiozome mass is usually 

 indistinguishable at this stage, though in some cases may still be 

 seen as a faintly staining mass near its original position. 



The next stage commences with the disappearance of the nuclear 

 membrane, which lades away earliest in the region of the pairs of centro- 

 somes (Fig. 176). Unlike Pentatoma, the nuclear membrane in Peri- 

 patus does not first become pulled out into a small cone projecting 

 towards the pair of centrosomes. When the nuclear membrane has 

 disappeared in the vicinity of a pair of centrosomes, a notable change 

 in the nucleus becomes apparent: the linin fibres converge towards 

 the centrosomes, as is shown clearly in Figs. 175, 176. Now the 

 linin fibres which undergo this movement are the distal linin fibres, 

 those which before had connected (as a part of the continuous linin 

 spirem) the distal end of one bivalent chromosome with the distal end 

 of another. Whether also the delicate fibrils, those which connected 

 chromosomes with the nuclear membrane, likewise converge towards 



