No. 3.] BUDDING IN PEROPHORA. 387 
The Branchial Stigmata. 
The branchial stigmata or gill slits are not formed until after 
the peribranchial cavity has been completely separated from 
the branchial sac. The first to appear lie far back towards the 
posterior end, but very soon they begin to break out in spots 
all over the sides of the branchial sac. 
The tendency to arise in vertical rows becomes apparent 
when only very few are present, but each slit is a separate and 
independent formation. I have never observed the origin of 
one slit from another, such as occurs in the larvae of ascidians. 
Fig. 5, Pl. X XIX, represents a stage when about eight slits 
have been formed on each side; as the anterior pouches of the 
peribranchial sacs grow further and further forward, new rows 
of slits are laid down along their free margins. 
The first indication of a gill slit is a small, circular, thickened 
area of the branchial wall, which at this spot becomes slightly 
evaginated until it touches the visceral wall of the peribranchial 
sac (Pl. XXXII, Fig. 29 a, g.s.v.). The cells of the latter at 
this point become thickened somewhat, and now a fusion takes 
place between the two walls; this is seen in Fig. 29 6 (g.s.7.). 
The opening, which breaks through the center of the fused 
patch of cells, is drawn out later in the long axis of the bud 
into a narrow slit, which is provided with cilia in the usual way. 
The upper part of Fig. 29 4 shows a slit just after the opening 
has been formed (4g.s.). 
The Branchial and Cloacal Orifices. 
The branchial orifice arises at a tolerably late stage, and is 
first indicated by a great increase in thickness of the ectoderm 
at a point opposite the extreme anterior end of the branchial 
sac. This thickened area becomes invaginated until the bottom 
of the pit touches the endodermal wall, and a complete fusion 
of the two soon takes place (Pl. XXXII, Fig. 28 a and 4, and 
Pl XX hice, 0¢6r.0;). The cells inthe center’ of ‘the 
fused area break down, and the cavity of the pharynx is put 
into communication with the outside. As is shown in Fig. 28 
