12 C. W. M. Poynter 
quote Keith (1895): “It is doubtful if one can legitimately con- 
strue this . . . as an atavistic tendency ; more probably it has no 
more morphological worth than the degree of interdigital web- 
bing.” ) 
Parsons (1902) has made an exhaustive comparative study and 
as a result of this thinks Keith is not warranted in drawing the 
above conclusion from the data available. He very strongly 
favors the idea that the comparative breadth or narrowness of 
the upper opening of the thorax is the chief determining factor in 
the arrangement of the trunks springing from the aorta. In re- 
spect to variations from the normal types among lower animals, 
all observers are agreed that they are unusual; from this fact he 
concludes: “These human anomalies are quite new and tentative 
attempts to meet some changed condition in man as progressive 
variations which may or may not become more common as time 
goes on.” 
For the sake of comparative interest in taking up the various 
classes of anomalies the animals having a similar condition as 
normal will be listed. 
$V 
I, IRREGULARITIES IN THE DEVELOPMENT OF THE TRUNCUS COM- 
MUNIS ARTERIOSUS 
The irregularities in this section will involve structures which 
belong in development to the heart, but I have chosen to consider 
them with the arteries because such cases are generally reported 
with vascular anomalies and because division from a purely de- 
velopmental standpoint is impracticable. The close association of 
this, or, more properly, these structures, with the general develop- 
ment of the heart is suggested by the fact that in almost all of 
these cases of anomalous development there are associated car- 
diac defects, the most frequent of these is incomplete interven- 
tricular septum. Later it will be necessary to review briefly the 
development of these structures when associated heart defects 
will be referred to in detail. 
240 
SS SSS aa 
