A Contribut. to the Erabryol., Life-history, and Classificat. of the Dicyemìds. 35 



Nematogens. That the axial cell of a Nematogen sometimes incloses 

 not less than nine large free nuclei , at other times only one , seems to 

 bave been entirely overlooked by Van Beneden. In the case of the 

 Rhombogens, on the other band, the plurinuclear condition was recog- 

 nized and partially accounted for. The occurrence of the same condi- 

 tion in Nematogeus remaiued a puzzle for a considerable time, as I 

 was unable by comparison of different stages of these individuals to 

 obtaiu any due to the origin of several nuclei. In the Rhombogens, 

 however, I bave been able to trace in a very complete manner the devel- 

 opment of this feature ; and I now regard its occurrence in Nematogeus 

 as one of the most conclusive evidences of a transition from one form 

 or condition to the other. 



A curious fact with respect to the number of these free nuclei may 

 bere bementioned, — a fact which can be explained on no other hypoth- 

 esis than that of a transition from the Rhombogenic to the Nemato- 

 genic condition. In Rhombogens the number may be either odd or 

 even; in Nematogeus it is invariably odd. The explanation of this 

 peculiar difiference will be seen when we come to trace the origin of the 

 nuclei in the Rhombogens. 



The differences in the forni of the axial cell are neither uniform 

 nor permanent. Under the head of diiferential characters, Van Beneden 

 States that the anterior extremity of this celi takes a rounded form in 

 Rhombogens, and a pointed form in Nematogeus. That this distinction 

 is not uniform is proved not only by my figures , but also by those of 

 Van Beneden. 



The Origin of Free Nuclei in the Axial Cell. 



In the fully formed embryo of D. moscliatum. (fig. 1, PI. 1), the 

 axial cell shows a single centrai nucleus [ne) and two primary germ- 

 cells [pg). 



This can not be said to be the originai nucleus of the axial cell, as 

 supposed by Van Beneden, since it is only one of the division-pro- 

 ducts of that nucleus , as will be shown in discussing the origin of the 

 germ-cells. 



Although it is not to be confounded with the nuclei which appear 

 later in the axial cell , I bave not always been able to distinguish be- 

 tweenthem, andtherefore, as a matter of convenience, include it among 

 the number of free nuclei, without intending thereby to express any 

 opinion on its relation to the axial cell. 



Sometimes this nucleus maintains its centrai position in the adult, 



