1 66 GRACE MEDES. 



The changes in the achromatic part of the nucleus during the 

 progress of the prophases are as considerable as those in the 

 chromatin. During the growth period the contents of the nuc- 

 leus, consist of the karyosphere, metaplasm granules and a coarse 

 linin net-work. The interspaces of this net-work are large and 

 clear, contrasting decidedly with the thick yellowish appearance 

 of the cytoplasm (Fig. 7). The appearing within the nucleus of 

 the cloud-like achromatic substance, which later forms envelopes 

 for the chromosomes, has already been described (Fig. 9). 

 During this process, the net-work may be seen unbroken be- 

 tween these darker masses ; but soon after the chromosomes 

 commence to form, the threads of this net-work begin to coalesce 

 and the meshes gradually become larger and more irregular 

 (Figs. io, n, 12). 



Thus we have the general appearance of the cell at about the 

 mid-prophase (Fig. 12). The nucleus occupies the central posi- 

 tion, while on each side, well out from its membrane, lies a cen- 

 trosome. Each of these consists of two dark granules situated 

 close together and surrounded by a very delicate aster. Within 

 the nucleus the threads of the net-work have coalesced con- 

 siderably and now form large and very irregular meshes. Several 

 small, dark masses, either metaplasm or nucleolar part of the 

 karyosphere, may be seen at various places in the nucleus. The 

 chromosomes have split longitudinally, broken into small round 

 unequal segments which have drawn together, and assumed the 

 typical form of the tetrad. Around these, the achromatic masses 

 which serve as envelopes, have contracted until no longer 

 discernible. 



The late prophase, judging from the comparatively few cells 

 to be found in this stage, endures but a short period. The 

 chromosomes continue to contract and to increase in density and 

 staining ability until they are reduced to but a small fraction of 

 their former size (Fig. 14, 15) when they lose their granular 

 nature and stain an even, intense black. Immediately after con- 

 tinued contraction, they lose also the tetrad form (Fig. 16), and 

 assume the shape of the " diplosome " described by Bouin 

 (Lithobins forficatus and Geophilus lineatus). At the breaking 

 down of the karyosphere, the accessory chromosome follows the 



