38 C. 0. Whitman 



Beneden. This cell is evidently uot identical with the cell m, figs. 104, 

 105, and the name germog-en, originally applied to both cases, may 

 therefore advautageously be restricted to the foi-mer ; while the entire 

 cell-group (g) maybecalledthe In fu sor igen, a terni iised;by Van Bene- 

 den as a synonym for Ehombogen. A somewhat later condition is seen 

 in flg. 110, taken from a young Rhombogen [Dicyemeiinea Eledones), 

 measuring 1.15 mm. The figure shows everything that was present in 

 the axial cell , namely , one Infusorigen , one paranucleus in) , and one 

 central nucleus [né] . The germogen presents, in addition to its nucleus 

 (^"), six to eight germ-cells. These germ-cells have very delicate out- 

 lines , which become tolerably distinct after remaining a few moments 

 inaceticacid; and they appear to lie within the" generative cell, or germ- 

 ogen. The peripheral cells of the Infusorigen are more numerous 

 than in stage 109, but they are still arranged in a single layer and form 

 a hemispherical envelope around the germogen. The paranucleus has 

 already attained a diameter considerably more than half that of the 

 large central nucleus, and still lies near the place of its origin. 



The still later form of the Infusorigen , seen in Fig. 111, differs 

 in no essential point from the one just described. The axial cell, of 

 which this figure shows only a part, contained one large central nucleus, 

 lying midway between two Infusorigens. With each Infusorigen were 

 associated mature germ-cells, cleavage-stages, young infusoriform 

 embryos, and a single paranucleus. The posterior Infusorigen, the one 

 seen in this figure, had five stages in front (only two given in the figure) 

 and three behind, consisting of a germ-cell with fusiform nucleus [ig), 

 an eight-cell stage, and an embryo (only outlined) , and behind all the 

 paranucleus [n] . The anterior Infusorigen had six stages in front and 

 five behind, the paranucleus lying before all. In this young Rhombo- 

 gen (2 mm long) we have then two proliferating centers (Infusorigens) 

 and three large free nuclei (the central nucleus and two paranuclei) . 

 Thus we find that, where there is one active Infusorigen only, there are 

 invariably two free nuclei, and no more ; and where two such Infusori- 

 gens are present, the number of free nuclei is always three. As every 

 Infusorigen is accompained by a paranucleus , and as the axial cell in- 

 closes a central nucleus at the outset , it is piain that in Rhombogens 

 the number of free nuclei will always exceed the number of Infusorigens 

 by at least one. Will the excess ever be more than one? The foUowing 

 Observation auswers this question. April 8 I found a Rhombogen (2mm 

 long) containing seven Infusorigens and nine free nuclei. Whence 

 came the ninth nucleus? The further history of the Infusorigen will ex- 



