510 PROCEEDINGS OF THE AMERICAN ACADEMY. 



pendent of the nucleus, but during this same period the nucleus is 

 also active. Aside from the evolution of the chromosomes and the 

 disintegration of the nucleolus, which have already been described, the 

 linin reticulum has also changed. Before the origin of the tetrads, a 

 considerable portion of the nuclear area is occupied by the diffuse 

 masses of achromatic material, which is so abundant as to obscure the 

 greater part of the nuclear framework (Figures, 9, 10). After the 

 chromosomes have arisen from the karyosphere, however, this diffuse 

 material is collected into smaller and denser masses, which closely sur- 

 round the chromosomes. Thus the linin network is rendered visible 

 and is seen to consist of a coarse, irregular reticulum (Figures 23-25). 

 At first this is granular and stains very diffusely. As the tetrads 

 condense to form the homogeneous chromosomes of the metaphase, the 

 network becomes coarser and more irregular. At the same time the 

 linin fibres begin to lose their granular appearance and to take on 

 the form of more definite threads, which retain the stains much more 

 strongly. Thus the nucleus in the late prophase presents the appear- 

 ance shown in Figures 26 and 27 (Plate 2). The^linin network is 

 much more irregular than in preceding stages, and at many places is 

 made up of distinct threads which have lost their granular structure. 

 In later stages they become more and more numerous and definite, 

 and the network becomes more and more broken up, although as yet 

 the fibres have no definite arrangement with reference to the poles of 

 the cells. 



The breaking down of the nuclear membrane and the formation of 

 the central spindle is a process involving considerable time, as is 

 shown by the numerous cells found in these stages. The nuclear 

 membrane disappears rather early, often before all of the chromosomes 

 have attained their homogeneous structure. The distinction between 

 nuclear and cytoplasmic material is not so early lost, however, and 

 may indeed be seen until the following anaphase. In the late pro- 

 phase the line of demarcation between nuclear material and cytoplasm 

 becomes very irregular, and shortly before the completion of the spin- 

 dle presents the appearance shown in Figure 28. The distinction 

 between nuclear sap and cell sap is shown by the much more trans- 

 parent appearance of the former. The linin at this time (Figure 28) 

 is all in the form of definite threads, and these are being oriented so 

 as to point toward the poles. 



In the later prophase this orientation of the linin fibres continues 

 until it eventually results in the formation of such a spindle as is repre- 

 sented in Figures 29, 30. The central spindle is derived from the 

 nucleus, as is shown by numerous stages in its formation. The chromo- 



