216 Mr. G. Lefevre on 
connexion with the stolonic partition remains stationary, and 
the rotation occurs on this as a fixed point. 
The pericardial rudiment, which is attached to the outer 
surface of the vesicle, is borne down towards the ventral 
side, and during the rotation is increasing in size by cell- 
division and by further additions of mesenchyme-cells. ‘The 
rudiment is at first solid, but some time before the rotation is 
completed a lumen appears in the centre of the mass of cells. 
When it reaches the ventral side, the pericardium is seen as 
an elongated closed vesicle formed of a single layer of cells 
and lying at the posterior end of the bud-rudiment imme- 
diately to the right of the median line; this is the definitive 
position. 
The heart is formed later in the usual way by invagination 
of the dorsal wall of the pericardium. 
At the stage shown in fig. 2 the rotation has proceeded to 
a considerable extent and the pericardial rudiment has acquired 
a lumen. 
Many transitional stages between figs. 1 and 2 have been 
examined, and from these it is evident that the rotation is due 
to a rapid growth and flattening of the cells forming the inner 
