64 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



becomes surrounded by a mass of archoplasm which increases rapidly in 

 volume, and by the time the centrosome has reached the vicinity of the 

 nucleus is of considerable size (Figs. 84, 86). During the earlier stages 

 of the migration the archoplasm is often of an irregular stellate shape 

 with long processes extending in various directions, but conforming to 

 the course of the cytoplasmic partitions. The centrosome during the 

 migration, which from the large number of stages found seems to occupy 

 a considerable time, becomes elongated in the direction of movement, 

 as shown in Figure 84. 



This elongation of the centrosome is in preparation for the formation 

 of the axial filament, the first stages in the production of which appear 

 during this migration. In Figure 85 such a stage is represented, show- 

 ing, I believe, that, while the axial filament arises in very close connec- 

 tion with the centrosome and possesses the same staining reaction, it is 

 not derived from the substance of this element. At the proximal end of 

 the entire structure, i. e. the end nearest the nucleus, lies the centrosome. 

 It is of the same shape and size as in the preceding stage (Fig. 84). At 

 the distal end of this elongated centrosome and fused indistinguishably 

 with it, arises a short somewhat moniliform fibre, which stains in the 

 same manner as the centrosome and is distinguishable from it only in 

 being at this stage narrower, less definite in outline, and tapering toward 

 its distal end. 



This compound structure is surrounded by a considerable amount of 

 ai'choplasm, which in a general way is of the same shape as the enclosed 

 parts (Fig. 85). The archoplasm takes a darker stain than the undiffer- 

 entiated cytoplasm of the cell, although of the same tint. The periphery 

 is more granular and denser than the part immediately surrounding the 

 forming axial filament. This latter is nearly homogeneous and much less 

 deeply stained. In other words, the archoplasm in the central portion 

 of the mass resembles more the condition which it assumes during the 

 early prophases of mitosis, when the astral rays are forming. 



By the time the archoplasmic mass containing the centrosome and the 

 growing axial filament has come to rest in the vicinity of the nucleus, 

 the axial filament is of considerable length and has the appearance of a 

 dense black thread which stands out with remarkable distinctness against 

 the orange-red background formed by the cytoplasm. The appearance 

 of the axial filament and its relations to the cytoplasm, archoplasm, and 

 centrosomes at this sta<;e are shown in Figures 86 to 89. The axial 

 filament, while always very distinct in doubly stained preparations, does 

 not as a rule possess at this stage a perfectly even and continuous outline. 



