LE P 1 DOS I ll K N PA P A D ( ) X A . 
89 
frilled on across the long axis of the bulbiis cordis (compare 
PL 5, figs. 5, 6 and 7, B. C. t., also Text- fig. 12a, b and 
C, B. G. t.). Thus a distinct bulging towards the left — tlie 
side away from the septum on the right wall — occurs, 
causing a marked external constriction ^ on this side between 
the proximal and middle parts of the bulbus (PI. 5, figs. 
5, 6 and 7, P.). As the bulging tapers more gradually 
towards its distal end, the third part of the bulbus is 
demarcated externally from the middle part by a less abrupt 
constriction^ on the right side (PI. 5, figs. 5, 6 and 7, D). 
This kinking and asymmetrical expansion of the middle pai-t 
of the originally comparatively straight bulbus results in tlie 
formation of the transverse segment, and the original right 
wall of this part adopts a transverse posterior (dorsal in terms 
of the adult) position. Similarly, that part of the right septum 
situated on the right wail of this region of the bulbus 
becomes correspondingly displaced, and extends, trans- 
versely to the long axis of the bulbus, along what has 
become the posterior (dorsal) wall of the middle part (Text- 
fig. 12c, B. B. 1.). If the two ends of a straight per- 
pendicular tube be approximated to one another so as to 
form a transverse kink about the middle of its length, it will 
be found that the right wall of the upper part is continuous 
with the upper wall of the transverse part, that is, with what 
was originally the right wall of the middle part. If now 
the transverse part expand forwards and upwards round its 
unextending upper wall, that wall — and any structure on its 
inner surface — will come to occupy a somewhat posterior 
position. This is apparently what happens in the develop- 
ment of the bulbus cordis of Lepidosiren. The gradual 
rotation of the definitive long axis of the heart into a 
position parallel with the long axis of the embryo must again 
be recalled, which, while the distal end remains fixed, swings 
1 Proximal and distal Knickungsfurche of the developing hiilbns 
cordis of Lacerta. — Greil, A., “ Beitriige znr vergleichenden Anatom ie 
nnd Entwickehingsgeschichte des Herzens und des Trnncns arteriosus 
der Wirbeltiere,” ‘Morph. Jahrb.,’ Bd. 31, 1903. 
