THE DEVELOPMENT OF THE HEART IN MAN. 
293 
left side of the heart, but it gradually moves to the right, and at the same time the 
orifice elongates in the same direction and becomes a horizontal slit. This alteration 
in position is due probably as much to a growth change in the atrium and in the 
ventricle, extending them to the left side, as to an actual transition in the position 
of the canal ; but there is also an actual extension of the orifice. At the same time 
the loose reticular tissue lying beneath the endocardium (seen in embryo 2Wl) 
becomes arranged into two definite masses which form the upper and lower cushions 
of the atrial canal. The extension of the orifice to the right side passes beyond the 
bulbo-ventricular septum and beyond the limits of the original ventricle, so that 
the right portion of the opening leads from the atrium into the chamber termed 
the bulbus cordis. The endocardial cushions of the atrial canal also extend in the 
same direction, and their right margins come into contact with the bulbar cushion 
lying on the right and dorsal aspect of this portion of the bulbus. 
This is a morphological fact of considerable importance, since it justifies the 
statement that the right atrium comes to open into the bulbus cordis. This fact is 
clearly shown in some of Born’s figures, but its morphological significance has not 
been pointed out. The upper portion of the bulbo-ventricular furrow remains as 
an infolding of the wall, marking the separation between ventricle and bulbus cordis. 
The contact of the bulbar cushion with the cushions of the atrial canal is well seen 
in the 6-mm. as well as in the 8- and 9-mm. specimens. 
It is convenient at this stage to turn to the bulbus cordis and to consider the 
part which it plays in the formation of the adult heart and its homologies. 
Considerable uncertainty exists as to the exact relationship of the bulbus cordis 
and Ventricular portions'respectively of the early embryonic heart to the chambers 
and vessels of the adult heart. 
Since the time of His it has been recognised that the ventricular portion of the 
early embryonic heart is in the form of a V, the two limbs being separated by a 
deep groove. 
In an early specimen described by His, similar to the youngest specimen of my 
series, such an arrangement exists. His gave to the left or descending limb the 
name of “ conus venosus,” and to the right or ascending limb the name of “conus 
arteriosus.” Such a terminology has obvious drawbacks, such as the fact that the 
name of conus arteriosus is used in adult human anatomy for a definite structure, 
not the same as the structure so named in the embryonic heart. 
Considerable light is thrown upon the significance of these portions of the heart 
by the study of the hearts in embryo as well as in adult fishes and reptiles. 
Boas, working on the heart in the Anamnia, recognised “ a clearly separated 
independent portion of the heart, which should be added as a third chamber to the 
auricle and ventricle,” and termed it in Anamnia the conus arteriosus. 
Kolliker and Born termed a similar portion of the heart in embryo Amniota the 
bulbus aortse, or bulbus arteriosus. 
