THE DEVP:LOPMENT OF THE VASCULAR SYSTEM. 



199 



While the double origin of the heart is characteristic of all amniotic Vertebrates 

 (Reptiles, Birds, Mammals), in all the lower forms the organ arises as a single anlage. In 

 the region of the fore-gut the two halves of the ccelom are separated by a ventral mesentery 

 which extends from the gut to the ventral body wall, and which is composed of two layers 

 of mesothelium with a small amount of mesenchyme between them. In the mesenchyme 

 a cavity appears and is lined by a single layer of flat (endothelial) cells. This cavity 

 extends longitudinally for some distance in the cervical region and with its endothelial 

 and mesothelial walls constitutes the simple cylindrical heart. On the dorsal side it is 

 connected with the gut by a portion of the mesentery which is called the dorsal meso- 

 cardium; on the ventral side it is connected with the ventral body wall by the ventral 

 mesocardium (Fig. 168). Thus the heart is primarily a single structure. The difference 

 between the two types of development is not a fundamental one but simply depends upon 

 the difference in the germ layers. In the lower forms the germ layers are closed in ven- 



Entoderm 

 Mesoderm (visceral) 



Heart 



Pericard. cavity 

 (coelom) 



Dorsal mesocardium 



Endothelium 

 'Mesoderm (parietal) 

 Ventral mesocardium 

 Ectoderm 





FIG. 1 68. Ventral part of transverse section through the heart region of Salamandra 

 maculosa embryo with 4 branchial arches. Rabl. 



trally from the beginning, and the heart appears in a medial position. In the higher 

 forms the germ layers for a time remain spread out upon the surface of the yolk or yolk 

 sac, and the heart begins to develop before they close in on the ventral side of the embryo. 

 Consequently the heart arises in two parts which are carried ventrally by the germ layers 

 and unite secondarily. 



The further development of the heart consists of various changes in the 

 shape of the tube and in the structure of its walls. At the same time the dila- 

 tation of the ccelom (primitive pericardial cavity) in the cervical region is of 

 importance in affording room for the heart to grow. In the chick, for ex- 

 ample, the tube begins, toward the end of the first day of incubation, to 

 bend to the right; during the second day it continues to bend and assumes 

 an irregular S-shape. This bending process has not been observed in 

 human embryos, but other Mammals show the same process as the chick. 

 In a human embryo of 2.15 mm. the S-shaped heart is present (Fig. 167). 

 The venous end, into which the omphalomesenteric veins open, is situated 

 somewhat to the left, extends cranially a snort distance and then passes 

 over into the ventricular portion. The latter turns ventrally and extends 

 obliquely across to the right side, then bends dorsally and cranially to join 

 the aortic bjulb which in turn joins the ventral aortic trunk in the medial 



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