THE DEVELOPMENT OF THE HEART IN MAN. 
301 
At the stage when the right and left ventricles become completely closed off 
from one another, this short-circuiting of part of the blood stream necessitates 
an alteration in the original peristaltic wave, so that the left and right ventricles 
may beat simultaneously, and it is when this separation has been affected that in 
my specimens there is the first indication of the presence of the atrio-ventricular 
bundle. 
Towards the expenses of preparing the models a grant was received from the 
Royal Society of London, and the expenses connected with the illustrations have been 
largely defrayed by a grant from the Carnegie Trust for the Scottish Universities. 
To these two bodies I wish to express my thanks for this assistance. 
Literature. 
The literature of the development of the heart is an extensive one, and need 
not be referred to in detail here, as adequate summaries are found in the Keibel- 
Mall Textbook of Embryology and in Tandler’s Das Herz, as well as in 
Hochstetter’s comprehensive bibliography in Hertwigs Handbitch der ver- 
gleichenden und experimentellen Entwickelungslehre. 
The works to which reference has been specially made, in addition to those 
above mentioned, are the following : — 
(1) Waterston, Journ. of Anat. and Phys., vol. xlix. 
(2) Thompson, P., Brit. Med. Journ., 1909. 
(3) Tandler, Anatomie des Herzen ; and Keibel-Mall, Manual of Hutiian Embryology, vol. ii. 
(4) Mall, Amer. Journ. of Anat., vol. xiii. 
(5) Keith, Journ. of Anat. and Phys., vol. xli. 
LIST OF PLATE ILLUSTRATIONS. 
Plate I. 
Fig. 1. Ventral view of wax-plate model of heart of 3-mm. embryo (2W1). 
Fig. 2. Ventral view of model of heart of 6-mm. embryo (SI). 
Fig. 3. Interior of the above model. 
Fig. 4. Dorsal wall of distal portion of the bulbus cordis in the same model. 
Fig. 5. Ventral view of model of heart of 8-mm. embryo (Bl). 
Fig. 6. View from the right of the interior of the right atrium and ventricle of the same model. 
Fig. 8. Interior of a model of the distal portion of the bulbus cordis of embryo M 1 (9 mm.) to show 
the endocardial cushion B, the root of the pulmonary artery and of the aorta. 
Fig. 9. Ventral aspect of model of the heart of 12'5-mm. embryo (S4). 
Plate II. 
Fig. 7. Caudal half of the same model divided horizontally. 
Fig. 10. View from the left of the same model after removal of the lateral wall of the left atrium. 
Fig. 11. Ventral aspect of model of 16-mm. embryo. 
TRANS. ROY. SOC. EDIN., VOL. LII, PART II (NO. 12). 48 
