LKPIDOSIREN PARADOX A. 
101 
the comparatively very short posterior auricular wall to the 
auriculo-ventricular junction. 
Septa of the Bulbus Cordis. — At Stage 30 a thicken- 
ing (Text-fig. 19, r.) is present on the ventral wall of each 
lateral ventral aorta, extending from a point between the 
origins of the common stems of the two anterior (third and 
fourth. Text-fig. 19, a. a. III and IV) and the two posterior 
(fifth and sixth. Text-fig 19, a. a. F and VI) afferent vessels, 
to tlie distal end of the bulbus cordis. 
In the bulbus itself, these ridges {B. R. 1. and B. R. 3) 
extend along the lateral walls towards its proximal end 
Text-fig. 19. 
a.a..v. arutvu. 
Section at Stage 30 transverse to the right ventral aorta at the 
point of divergence of the common stem of the third and fourth 
from that of the fifth and sixth afferent aortic vessels. 
a. a. Ill and IV. Common stem of third and fourth aortic 
arches. a. a. V and VI. Common stem of fifth and sixth 
aortic arches, r. Ridge of thickening on ventral wall between 
the two vessel stems. 
(Text-fig. 9c, B. R. 1 and B. R. 3). The right ridge 
{B. R. 1) extends as far as the distal end of the proximal 
part of the bulbus, where it tapers off and is lost; the left 
ridge {B. R. 3) extends only about a third of the way along 
the left wall of the bulbus. In the proximal part of the 
bulbus a ridge {B. R. A.) also appears at the same time on 
the anterior (ventral in the adult) wall (Stage 31,' Text-fig. 
20b, c and d, B. R. A.). 
Presently, about Stage 32, a general cellular proliferation 
occurs round the circumference of the whole length of the 
bulbus — except on the expanded middle part — which results 
at its distal end in the temporary appearance of two addi- 
tional ridges (fig. 20a, B. R. 2 and B. R. 4) between the 
