THE HYPOMERE (LATERAL PLATE MESODERM) 235 



heart. There is thus a reversal in the original anterior-posterior re- 

 lations of the heart in respect to the embryo. The extremities of the 

 heart, those parts which are held in place by the dorsal mesocardium, 

 are the posterior sinus venosus and the anterior truncus or bulbus 

 arteriosus. Anteriorly the bulbus arteriosus gives rise to three pairs 

 of arteries (aortic arches) to each of the three pairs of external gills. 

 Later a fourth pair is added. In the frog there are no aortic (arterial) 

 arches in either the mandibular or hyoid (first or second visceral) 

 arches. 



Before the development of the heart has progressed this far (i.e., 

 the 4 mm. stage) there appears a pair of blood vessels in the ventro- 

 lateral splanchnic mesoderm known as the vitelline veins. These are 

 associated closely with the yolk mass, hence the name "vitelline" from 

 the vitellus (yolk). They probably are derived, as is the heart, by 

 a separation of yolk endoderm and ventral mesoderm. They grow 

 anteriorly by the merging of blood islands or lacunae until the veins 

 merge, anterior to the yolk and liver mass, and become the fused 

 vitelline veins. This point of fusion of the vitelline veins is called the 

 sinus venosus from the very beginning. It represents the most poste- 

 rior limit of the heart. The part of the heart into which the sinus 

 venosus empties is the atrium or the undivided embryonic auricles. 

 The atrium and subsequent auricles remain thin-walled and become 

 very elastic. Shortly (i.e., at the 7 mm. body length stage) there grows 

 down from the roof of the atrial chamber a longitudinal sheet of endo- 

 cardium which is known as the interatrial septum. This divides the 

 atrium into a right and left auricle, and the sinus venosus now leads 

 into the right auricle. Eventually the left auricle will receive blood 

 from the pulmonary veins. 



The ventricle or anterior portion of the original heart tube becomes 

 very thick-walled and muscular and is never divided longitudinally 

 into two as it is among mammals. There is a transverse division 

 groove between the atrium and the ventricle at the folds in the in- 

 verted "S" coil just described. The ventricle is therefore folded cau- 

 dally and ventrally to the atrium and leads into the truncus arteriosus 

 which is suspended to the dorsal body wall by the permanent dorsal 

 mesocardium at this level. The truncus arteriosus becomes partially 

 subdivided by a vertical and longitudinal septum (the spiral valve) 

 and gives rise to the paired ventral aortae which are connected with 

 the various aortic arches. 



