363 | 
The ribs on the third and fourth vertebr& are similar in length 
and thickness. They are at first cylindrical in shape, but later in- 
crease in breadth at their proximal and distal extremities more than 
in the middle; and, since the distal extremity of the diapophysis be- 
haves in the same way, there appears, in the second of the stages 
figured (Fig. 2), a nodal swelling at the articulation of the rib on to 
the transverse process. Curiously enough v. JHERING (25, p. 299, 
Fig. 1) actually saw this swelling and introduced it into his sketch 
of the first few vertebre of a late embryo of Pipa without noticing 
the articulation which exists here; for he would most assuredly have 
mentioned such an interesting feature, had he observed it. The second 
pair when first developed are parallel to the first, but they slope 
somewhat more posteriorly when ossification begins (Fig. 1), and have 
a very considerable backward slope before the metamorphosis is com- 
Fig. 1, Fig. 2. 
Fig. 1. Pipa americana. Embryo from the back of the mother. (Distance from 
snout to cloaca, 10 mm. Length of tail, 10 mm. Length of hind limb when extended, 
7,5 mm. Length of fore limb, extended, 4,5 mm.) Vertebral column, ventral view. 
Fig. 2. Pipa americana. Ripe embryo, (Tail completely absorbed. Distance 
from snout to cloaca, 14 mm. Length of hind limb when extended, 10,5 mm. Fore limb, 
6 mm.) Vertebral column, ventral view. d. diapophysis of vertebra 6. n. notochord. 
r. rib of vertebra 4, sd. sacral diapophysis. wur. ventral constituent of the urostyle. 
3. centrum of vertebra 3, seen through the transparent notochord. 7. centrum of verte- 
bra 7. 9, foramen for the ninth spinal nerve, 
24% 
