368 University of California Puhlications in Zoology [^o^. 18 



Division of the Macronucleus 



About the time that the micronucleus begins to migrate from its 

 position in the indentation of the macronucleus, there appears at each 

 end of the macronucleus a very conspicuous band. Each band is made 

 up of two distinct areas : one a very heavily stained part, and joining 

 to it on the side toward the end of the nucleus a non-staining portion, 

 apparently almost, if not entirely free from chromatic material (pi. 

 15, fig. 14). As stated above the dark portion of the band stains very 

 heavily and if overstained appears almost homogeneous. However, by 

 careful destaining it can be shown that this area is in reality one in 

 which the chromatin granules are very closely packed together. This 

 part of the band was first noticed by Stein (1859) in E. patella, but 

 from then on until Griffin (1910) described it for E. worcesteri no 

 one seems to have noticed it. To this band Griffin (1910) gave the 

 name "solution plane" and suggested that in this region the chromatin 

 was in solution in the karyolymph. Immediately distal to this solution 

 plane is another band which is called by Griffin (1910) the "recon- 

 struction plane." This part of the nucleus takes no stain except for 

 a few granules (pi. 15, fig. 16), the whole area appearing as though 

 the chromatin material had been removed, leaving only the linin net- 

 work of the nucleus. Taken together Griffin calls the two areas the 

 "reconstruction band." 



The two planes are sometimes about the same thickness Irat in the 

 majority of cases the solution plane is thinner, being about two microns 

 wide while the reconstruction plane is about three microns wide. The 

 fine fibrils which run through the reconstruction plane seem to be 

 connected to the ragged edge of the solution plane on one side and to 

 the chromatin granules of the reorganized area on the other. The 

 granules which are to be seen in the light area indicate that a re- 

 organization process is going on in the compact area, and that the 

 reformed granules pass along the fibers of the light area to the distal 

 part of the nucleus, which is made up of the reconstructed granules. 

 These two parts of the nucleus, the original nucleus and the two distal 

 parts, are sometimes similar, that is the granules are about the same 

 size and have a similar staining reaction, but in other specimens the 

 granules in the reorganized area are larger and stain more heavily 

 than those in the original unchanged part of the nucleus. 



The two reorganization bands migrate toward each other until they 

 meet at the middle of the nucleus. During about half of the period 



