54 PETER OKKELBERG 



groups at opposite poles. The spindle is still distinct. The cell 

 is constricted in the middle and the two daughter cells are about 

 to separate. Sometimes a deeply staining strand or thread 

 remains between the two chromatin masses after they are some 

 distance apart (fig. 49) . In a slightly later stage a distinct mid- 

 body is seen along the line of separation (fig. 50). Soon after 

 this the two daughter cells separate, a new nuclear membrane 

 forms around each daughter nucleus, and the two nuclei are in a 

 stage of reconstruction. The two daughter cells are smaller 

 than the mother cell (fig. 51). Small chromatin masses reap- 

 pear and are united by connecting achromatic threads. At least 

 one nucleolus occurs in each cell. After division the cells enter 

 a period of growth until they have assumed the structure and 

 size of the mother cell. 



Discussion. There is nothing strikingly peculiar in common 

 mitosis of the germ cells of the lamprey and only a few obser- 

 vations need further comment. First, the process of division is 

 probably not very rapid. This statement is based on the fact 

 that the number of germ cells does not at any time increase very 

 fast. From the number of germ cells in the mitotic phase at any 

 one tune, it is evident also that the karyokinetic period must be 

 rather long, probably occupying several days. 



The chromosomes are more or less rounded and stand out most 

 clearly in the middle prophase (fig. 44) . The number of chromo- 

 somes is very large, and they often give the impression of being 

 in pairs. This may mean one of two things: either that the 

 chromosomes have already divided during the prophase stage 

 before they have reached the equatorial plate or that the mater- 

 nal and paternal chromosomes remain associated during the 

 prereduction stages. The latter was found by Chubb ('06) to 

 be the case during the multiplication period in the germ cells of 

 Antedon. Stevens ('07, '08) found that in Diptera a pairing 

 of the chromosomes took place in germ cells far removed from 

 the reduction stages and occurred in connection with each oogo- 

 nial and spermatogonial division. Metz ('16) has reinvestigated 

 the problem in Diptera, and from a study of about eighty species 

 has come to the conclusion that in somatic, as well as in germ 



