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PROFESSOR D. WATERSTON ON 
Development of Nodal Tissue and Connecting Tissue 
(Atrio-ventricular Bundle). 
Nodal tissue possessing the adult characteristics cannot be definitely recognised 
in the developing heart at the stages described, probably because its histological 
features are not sufficiently differentiated. 
The right venous valve from an early stage, however, shows histological charac- 
ters which distinguish it from adjacent portions of the heart wall, such as a loose 
texture and vacuoles, and this may precede the formation of the sinu-atrial muscle 
of Keith and Hoch. 
Neither does the atrio-ventricular bundle present in the young embryonic heart 
the features which allow it to be recognised in the adult. 
The most that can be said, from the specimens examined, is that from an early 
stage a structure can be identified which has special histological characters, and which 
lies in the position afterwards occupied by a part of the atrio-ventricular bundle. 
This structure is recognisable in the 16-mm. embryo, and is very distinct in the 
20-mm. embryo as well as in later specimens. 
It consists of a rounded mass of cells whose nuclei stain rather darkly and are 
more closely packed together, surrounded by a ring of very loose tissue. 
This structure is most clearly seen in the upper part of the muscular inter- 
ventricular septum, close to the apex, and it appears to run into and to be connected 
with the cushion tissue of the atrio-ventricular canal. The peripheral connections 
of this structure with the walls of the ventricles cannot be discovered, except 
that on the right side it appears to pass towards the origin of the “ moderator 
band.” 
This band is in part covered by the continuation of the bulbar cushion to the 
ventricular septum, and it is interesting to find that the path in the adult heart 
of the principal limb of the atrio-ventricular bundle lies in tissue immediately 
underlying a layer derived from cushion tissue. 
The histological characters of this mass have been fully described and figured 
by Mall, and his figures represent accurately the condition found in my specimens 
from 20 mm. onwards. 
Towards the atria I have not been able to trace definitely any connections of 
this bundle, beyond the fact that it runs into or below the cushion tissue of the 
atrio-ventricular canal and joins with the right and left venous valves. 
It may here be pointed out that the process of development of the heart as 
described here shows a short-circuiting of part of the blood stream. In the early 
stages the blood passes, from atrium to ventricle, and thence to bulbus cordis. 
The alteration in the position of the atrial canal allows part of the blood stream 
to pass from the right division of the atrium into the bulbus cordis and so to the 
truncus arteriosus, without passing through the primitive ventricle at all. 
