48 C. 0. Whitman 



entire axial cell. Germ-cells (acetic acid) varied between .006 mm and 

 .012 mm; the nuclei of the larger cells averaged .004 mm. 



A second primaiy Nematoden, 1 .60 mm long, inclosed no embryos; 

 the germ-cells measured from .004 mm to .010 mm, the nuclei from 

 .003 mm to .006 mm. The germ-cells were not numerous, and occu- 

 pied only a little more than the middle third of the axial cell. 



A third primary Nematogen, 3 mm long, inclosed three full}'^ devel- 

 oped embryos and germ-cells varying between .006 mm and .015 mm 

 with nuclei from .003 mm to .005 mm. Osmic acid. 



A fourth primary Nematogen contained one embryo and germ-cells 

 which varied from .006 mm to .014 mm. The larger cells were paler 

 than the smaller ones, and a few inclosed two nuclei. Acetic acid. 



A fifth primary Nematogen, 6.55 mm long, contained a few embry- 

 onic stages, and numerous germ-cells .006 mm — .014 mm in diam., 

 with nuclei .003 mm — .005 mm in diam. Osmic acid. 



A Rhombogen, 4 mm long, contained two Infusorigens with embry- 

 onic stages, and three free nuclei. 



The mature germ-cells measured .015 mm and their nuclei .008 mm. 

 Acetic acid. 



2. Apr. 9. A primary Nematogen .75 mm long. The entire Con- 

 tents of the axial cell seeu in fig. 101, PI. 5. Here are two large round 

 pale cells vg (.014 mm in diam.ì and smaller cells that are much more 

 coarsely granular. As the large pale cells are also found in Nematogens 

 that contain embryos and numerous germ-cells, it seems to me probable 

 that they are the mature germ-cells, and that they arise from the 

 smaller and darker cells by simple grow^th. 



There is not then much difference in size between the two kinds of 

 germ-cells in this species. The nuclei of the infusorific germs are however 

 here , as in ali the species I bave examined, conspicuously larger than 

 those of the vermifìc class. 



Dicyema truncatum Apr. 14. Fig. 99, PI. 5, shows the contents of 

 the axial cell of a young Nematogen .3mm long. Besides the centrai 

 uucleus (we) , there are live round germ-cells with small nuclei and a 

 large fusiform celi with large nucleus. As I find in another individuai 

 (.45 mm long) only the centrai nucleus and two large fusiform germ- 

 cells, I infer that the small germ-cells in fig. 99 bave arisen by division 

 of large fusiform cells. 



The in fusori gen. — Having traced the history of the Infusori- 

 gen from its origin to its dissolution , we may now consider briefly the 

 question which this history raises in regard to its siguificance. Is it a 



