360 
Pipa americana. 
In examining the vertebral column of a moderately advanced 
embryo of Pipa, what strikes one more forcibly than anything else 
is the breadth of the vertebre and the capacity of the spinal canal 
as compared with the size of the notochord. These features, which 
give such a characteristic appearance to the embryonic spine, are not 
long in making their appearance. The neural arches develop first, 
and form a regularly arranged series of cartilages on the dorso-lateral 
regions of the notochordal sheath. These cartilages grow upwards, 
and finally inwards to form the roof of the spinal canal. The com- 
pletion of the roof is very much delayed, and even in ripe embryos 
the right and left halves of the neural arch have not yet united dor- 
sally. The neural arches, being the oldest constituents, are the first 
parts of the vertebre to undergo ossification. Their bases remain 
separate from one another for a considerable time, and the centra 
when first formed are not sharply marked off from them. The neural 
arches soon become so broad in proportion to the width of the noto- 
chord that their ventral ends pass almost horizontally inwards towards 
the middle line, and the centra serve to fill up the gap between these 
basal ends, and thus to complete the ventral portion of the neural hoop. 
As first observed by Stanntus (40, p. 15) the vertebrae of Pipa 
form on the dorsal surface of the notochord, and not in rings around 
it as in Rana and Bufo. The vertebral development is thus of that 
type to which GEGENBAUR some years later applied the name “epi- 
chordal” (17, p. 33). The centra, which Stannius failed to recognize, 
are flat plates of cartilage, rectangular in outline and closely applied 
to one another in a longitudinal series (3, Fig. 1). Ossification in 
the centra is rather tardy. It commences in two areas simultaneously 
in each centrum, one towards the anterior and the other towards the 
posterior end; but in the first few vertebre the deposition of lime 
salts is more diffuse. The process begins at about the fourth or fifth 
vertebra, so that in ripe embryos the calcification is denser here than 
in the other centra (see Fig. 2). In the stage shown in Fig. 1, the 
notochord is already considerably collapsed in the presacral region, 
but it still retains its cylindrical and unwrinkled exterior in the tail. 
In the later of the embryos figured (Fig. 2) the notochord has been 
entirely absorbed. 
The original view of SCHNEIDER (87, p. 257), MECKEL (28, p. 386) 
and Cuvier (11, p. 399) that the first apparent vertebra of Pipa 
represents the first two of other Anura, and that the transverse pro- 
cesses which it bears are really those of the second vertebra, is en- 
dorsed by Stannius (39, p. 130, and 40, p. 16) and Owen (32, p. 49) 
