GERM-CELL HISTORY IN THE BROOK LAMPREY 47 



The germ cells of the lamprey have not been seen to divide 

 except by mitosis. The nuclei of the cells are always spherical 

 and never have the lobulated appearance which seems to be 

 characteristic of the nuclei of the germ cells of amphibians. 

 Several investigators (Vom Rath, '91; Meves, '91; McGregor, 

 '99) have recorded amitosis as taking place in the germ cells of 

 the latter group, but it is possible that the appearance of ami- 

 tosis comes from sections through lobulated nuclei. Recently 

 Macklin ('16) has made a study of apparent amitotic phenomena 

 in heart cells of chick embryos growing in vitro, and has found 

 that nuclei divide by bilateral and unilateral constriction, but 

 that afterward the parts of the nuclei recombine and divide by 

 normal mitosis. It may be that in some animals similar 

 processes take place in the germ cells. 



In the lamprey the cells in the same cyst do not always divide 

 simultaneously, though generally, if the cells in a cyst are divid- 

 ing, most of them are in one phase or another of mitosis. At the 

 same time the cells in neighboring cysts may all be in a resting 

 stage. Since the germ cells do not multiply very rapidly, there 

 is a considerable period of rest between successive simultaneous 

 divisions in a cyst. The synchronous division of the cells of a 

 cyst may be due to their close relationship, all being derived 

 from one cell and having been subjected to similar environmental 

 influences, or to some stimuli from outside sources, the effect of 

 which is limited to a single cyst. King ('08) found that in Bufo 

 the cells of a cyst did not divide simultaneously. According to 

 Jorgensen ('10), this is also true in Proteus. Bouin ('01), how- 

 ever, describes the cells of a cyst as dividing simultaneously in 

 the frog. It seems likely that there may be considerable variation 

 in this respect in different forms and even in the same species. 



Since, in the lamprey, most of the cysts are broken up from time 

 to time by the inward migration of follicular cells, it is impossible 

 to say whether or not there is a constant number of divisions of 

 the indifferent germ cells. In some cases the cysts become 

 very large and contain hundreds of cells, but it is not certain 

 that only these came from one primordial germ cell. In most, 

 if not in all cases, each primordial germ cell gives rise to many 



JOURNAL OF MORPHOLOGY, VOL. 35, NO. 1 



