No. 3.] BUDDING IN PEROPHORA. 393 
patch; this is the rudiment of the pericardium, which is the first 
organ to make its appearance. In Pl. XXXII, Fig. 25 a, which 
is drawn from a frontal section, an extremely early stage is 
shown at gc.r. That the rudiment is formed by the coming 
together of free amoeboid cells of the blood 1 believe there is 
no reason for doubting. At the stage represented in this 
figure, the similarity between many of the cells scattered freely 
about in the space between the ectoderm and endoderm and 
those which form this cell mass is perfectly apparent. There 
is certainly not the slightest evidence that the wall of the inner 
vesicle evaginates or its cells proliferate at this point; the line 
of demarcation between the two structures is distinct through- 
out, and shows no interruption in its continuity. 
The rudiment does not long remain of one layer, but by the 
addition of other cells and by active cell division it soon becomes 
thicker and more compact (Fig. 25 4). The cell boundaries 
are gradually lost, and the solid mass is now firmly attached to 
the wall of the vesicle (Fig. 25 ¢). The rudiment, which has 
now an elongated form, is not in a horizontal position, but 
posteriorly is at a higher level than anteriorly. 
When the shifting of the inner vesicle begins, the clump of 
cells is borne passively down towards the ventral side, but long 
before it has reached its definitive position a cavity has appeared 
in its center, around which the cells become arranged in an 
epithelium to form an elongated closed sac (Figs. 25 @ and e). 
The position of the rudiment at various stages during its 
descent has already been observed while considering the 
displacement of the inner vesicle from Pl. XXX, Figs. 9, 10, 
Pieandy bls XXX, Figs, 20) 24: 
About the time that this change in position is accomplished 
the pericardial sac loses its attachment to the branchial wall, 
and grows considerably longer and wider. The cells composing 
the sac become very much flattened and attenuated, except in 
the dorsal wall, which is soon folded in longitudinally to form 
the heart in the usual way (Fig. 24, d.w.pc.s.). The peri- 
cardium in its definitive position is placed under the posterior 
floor of the pharynx, just to the right of the median line. It 
is not horizontal, z.¢., parallel with the surface of the stolon, 
