174 ELEMENTS OF MAMMALIAN ANATOMY 



venous portion is transformed into the atrial or auricular portion 

 of the adult heart and the arterial into the ventricular portion. 

 It is interesting to note that the heart in its embryonic develop- 

 ment passes through a stage in which there is a single circulation, 

 resembling the condition found in lower gill-breathing aquatic 

 vertebrates where only impure blood is found in the heart. 

 Also that an arterial arrangement is set up as though for a gill- 

 breathing animal. It is only later when the true lungs are 

 developed that the heart becomes four chambered and a double 

 circulation characteristic of higher vertebrates is established. 

 In connection with this change in the structure of the heart 

 there are certain transformations in the arrangement of the 

 arterial system changing it from the gill-breathing type to that 

 of the lung-breathing, a change that will be better understood 

 when compared with the phylogeny of the aortic arches shown 

 in Fig. 83. 



DEVELOPMENT OF THE AORTIC ARCHES 



The aortae and aortic arches are developed from an arterial 

 plexus in the region cranial and dorsal to the developing heart. 

 The aortge are developed from a plexus in the area vasculosa 

 along the lateral edge of the myotomes. This capillary plexus 

 appears to be developed in situ by the fusion of intercellular 

 mesenchymal spaces or clefts. According to Coulter ('09) 

 only the first aortic arch is complete in a cat embryo of 3 mm., 

 although there are indications of the second and third arches 

 (Fig. 87a). These are completed at the time an embryo has 

 reached the length of 4.5 mm. and the fourth arch has begun 

 to be formed (Fig. 876). In a 5 mm. embryo the first arch 

 begins to degenerate, while the fourth is completed and the 

 sixth is begun (Fig. 87c). It is doubtful whether the fifth 

 arch is ever completed in the cat although rudiments of it 

 can be seen at a stage slightly older than five millimeters (Fig. 

 Sjd). Before the six millimeter stage is reached the sixth arch 

 is usually completed and gives origin to the pulmonary arteries 



