THE DEVELOPMENT OF THE VASCULAR SYSTEM. 255 



necessarily imply the absence of the heart in the affected twin, for the latter 

 may develop to a considerable degree and possess a functionating heart. 

 On the other hand, the affected twin may be only an amorphous mass of 

 tissue which derives its total blood supply through the agency of the stronger 

 twin's heart. Or there may be any intermediate form between these two 

 extremes. The point is that the acardiac monster (acardiacus) derives its 

 blood wholly or in part through the agency of the stronger heart. A further 

 discussion of acardiac monsters and their possible explanation will be found 

 in Chap. XX. 



DOUBLE HEART. But one or two Ceases of a double heart in a single 

 human foetus have been recorded. In some of the lower forms (chick) it 

 occurs more frequently. The explanation is probably to be found in the 

 double origin of the heart in Amniotes (p. 196). 



ANOMALOUS POSITION OF THE HEART. Congenital anomalies in the posi- 

 tion of the heart are rare. Dextrocardia (heart on the right side) is almost 

 invariably associated with changes in the position of the viscera (see trans- 

 position of the viscera, page 304) . In the condition known as ectopia cordis, 

 the heart, with the pericardium, protrudes through a cleft in the ventral 

 wall of the thorax, the cleft being probably due to an imperfect fusion of the 

 two sides of the body wall in that particular region. 



ANOMALIES OF THE SEPTA. The most frequent anomaly in the atrial 

 septum is the persistence of the foramen ovale. The entire foramen may 

 remain patent, or, as is more frequently the case, a smaller opening may 

 persist between the ventral (anterior) border of the foramen and the valve of 

 the latter (p. 203). 



The atrial septum may be wholly lacking, but this always occurs in con- 

 junction with other defects. It sometimes happens that the primary atrial 

 septum (septum superius), which grows from the cephalic side of the common 

 chamber, fails to fuse with the septum of the atrio-ventricular aperture (p. 

 203 and Fig. 171). 



Defects in the ventricular septum occur less frequently than in the atrial 

 septum. It may happen that the cephalic (upper) border of the ventricular 

 septum fails to fuse with the septum which divides the aortic trunk and bulb 

 into the aorta and pulmonary artery. This affects the cephalic (upper) part 

 of the septum sometimes called the pars membranacea (p. 204 and Fig. 174); 

 and since the defect is situated near the opening of the aorta it brings about 

 the so-called "origin of the aorta from both ventricles." Stenosis of the 

 pulmonary artery usually accompanies this condition. Rarely is there a 

 deficiency in the caudal (lower) part of the ventricular septum. Complete 

 absence of the ventricular septum may occur, and along with it also an 

 absence of the atrial septum, so that the heart is simply two-chambered; or 



