222 Papers from the Marine Biological Laboratory at Tortugas. 



distinct whatever connection there may be between them is gradually lost 

 (figs. 36 and 39). 



As the distal centrioles move away from the base of the cell, there is 

 formed between each pair of daughter centrioles an axial fiber (figs. 34, 35, 

 etc.). These fibers are comparatively stout, more so than the flagella, and 

 they stain very intensely with iron haematoxylin. In common with all the 

 other centrosomal structures, they are brought out best in those portions 

 of the tissue where the action of the acetic and osmic acids has not been too 

 strong. In their journey across the cell, spinning out the axial fibers behind 

 them, the distal centrioles come to pass through the ring of mitochondria 

 (figs. 36 and 39). As they approach this ring they undergo a lateral com- 

 pression in two opposite directions, so that they draw closer together and 

 eventually fuse to form a solid, very flatly oval plate, whose greatest diam- 

 eter is no less than that of the group of proximal centrioles at the base of 

 the cell. The compression of the distal centrioles with the corresponding 

 flattening of the bundle of axial fibers is shown in figures 36 and 39. It is 

 shown to better advantage, perhaps, in figures 37 and 38, which represent 

 two consecutive cross-sections through a bundle of axial fibers at about the 

 stage shown in figure 39. The first of these cuts the axial fibers near the 

 base of the cell and the second right through the ring of mitochondria; they 

 bring out the difference in contour of the bundle of axial fibers at those 

 two points and give some idea of the enormous number of fibers which 

 constitute the bundle. 



In front of the advancing centrosomal plate lies the area of dense 

 material which was mentioned before and which is now becoming more and 

 more evident (figs. 36 and 39). Around the anterior portion of the bundle 

 of axial fibers lie other masses of denser cytoplasm, the inner margins of 

 which are lined by the mitochondria (fig. 38). At this stage, or a little 

 later, in many instances the bundle of axial fibers becomes somewhat twisted 

 and the centrosomal plate is thrown a little out of its horizontal position. 

 Further growth of the axial fibers is accompanied by a further flattening in 

 its anterior portion and a more complete condensation of the centrosomal 

 plate, so that the whole structure, when viewed from the side, has a sharply 

 pointed appearance (fig. 40), while its upper and lower surfaces are as broad 

 anteriorly as they are at the base of the cell (fig. 41). The dense substance 

 in front of the centrosomal plate is pushed on ahead, while the other masses 

 of dense cytoplasm, together with the mitochondria, spread out along the 

 bundle of axial fibers behind the centrosomal plate, forming a sort of sheath 

 around the former. The mitochondria are shown in longitudinal section 

 in figure 40 and in surface view in figure 41. 



As is indicated in figure 40, the spreading of the mitochondria and of 

 the dense cytoplasm associated with them results in their separation into two 

 parts, the posterior one of which comes to form a secondary ring around the 

 base of the bundle of axial fibers. As before, this secondary ring is com- 

 posed of the dense and homogeneous cytoplasm, the inner surface of which 



