The Morphology of the Tadpole oj Xenopus laevis. 
253 
running forwards to the anterior edge of the supposed first bar from the 
supposed second. 
2. The first aperture may also be explained as a vestige of the 
separation between the bases of branchials 2 and 3. The arrangement of 
the pouches — one starting in front, one behind, and one at it — would 
favour such a view. The artery passing through this aperture must then 
be regarded as a *'new" development. 
3. In other amphibia there are at most four bars — not five. 
Thus, although at first sight there seems to be five bars, I have come to 
the decision that there are really only four. 
With regard to the question as to the origin of the columella — the 
branchial bars are joined dorsally to the auditory capsule by a sheet of 
dense connective tissue in which there are several centres of chondrifica- 
tion, and there are also in this sheet two distinct bars of cartilage (PB 
1 and 2), one attached to the auditory capsule just above the fenestra 
ovalis, the other joined to the extreme posterior region of the capsule. It 
seems unlikely that the first of these bars (PB 1), which is exactly in the 
position that the columella will later on be in, should totally disappear and 
be replaced by an upgrowth of the hyoid. It seems more probable that it 
is retained, and forms at least the stapedial portion of the columella. The 
two cartilaginous rods (PB 1 and 2) are, as mentioned above part of a 
continuous sheet of dense connective tissue, which in some places shows 
distinct chondrification and which stretches from the outer edge of the 
branchial plate to the auditory capsule and forwards to the quadrate (Q). 
Such a plate can only be considered to be the fused pharyngo-branchial 
elements of the branchial bars, and the stapedial portion of the columella 
is therefore probably of branchial and not of hyoidean origin. 
Blood Vasculae System. 
Vejious System.— From the anterior part of the body the blood drains 
into the sinus venosus through three pairs of veins. The lingual vein 
(Fig 8 Lv) runs from the muscle (M4 Fig. 1) down to the skin (Lv 
Fig. 3). The external jugular vein (EJv Fig. 8) comes from the floor of 
the mouth (EJv Fig. 3). The precaval vein (pcv Fig. 8) comes from 
the pronephros through which it is continuous with an internal jugular 
vein (IJv Figs. 8 and 3 ; 2 Fig. 2). The internal jugular vein in the 
region shown in Fig. 2 gives off a branch which passes between the gland 
Th and slit S, and receives branches from the roof of the pharynx. The 
internal jugular also receives a number of branches from the brain and 
dorsal body-wall. The precaval just after leaving the pronephros receives 
a vein (Bv Fig. 8) from the outer edge of the filtering apparatus. The 
