232 STRUCTURE OF THE VERTEBRATES 



2. Development of the Mammal Heart. The heart begins as 

 ^ fusion of the two vitelline veins, and is a single straight 

 tube. As the heart region enlarges and becomes more muscular 

 it assumes an S-shape, the posterior (auricular) end moving 

 dorsally. The ventricular portion becomes heavily muscled and 

 forces the blood into the ventral aorta as it is received 'from the 

 auricles. Further growth of the ventricle forces it posteriorly, so 

 that the heart soon has the appearance of the auricles being 

 anterior and the ventricle posterior in position. If the student 

 understands the twisting and growth of the heart, the primitive 

 morphological relationships w411 be clear. 



As the septa in the heart develop the ventricular septum com- 

 pletely cuts this chamber into two; but the auricular septum is 

 left incomplete until after birth, this opening (the foramen ovale) 

 between the two auricles being of functional benefit in embryonic 

 circulation, for the blood is aerated in the placenta rather than 

 in the lungs. If the foramen does not close soon after birth the 

 blood is mixed as it enters the aorta and a cyanotic condition 

 results in the individual. The so-called "blue babies" are persons 

 affected by this anomaly. 



3. Aorta and Aortic Arches. The ventral aorta is the vessel 

 from the heart passing out of the pericardial cavity along the 

 ventral side of the pharynx. In the early embryonic condition of 

 all vertebrates the aortic, or branchial, arches pass dorsally to 

 the paired dorsal aortae. The typical number of arches is six, 

 although the cyclostomes and most primitive elasmobranch fishes 

 have a greater number. Beginning with this primitive condition 

 there are many modifications in the groups of vertebrates, but 

 the ontogenetic and phylogenetic histories of the classes are 

 closely parallel. 



The elasmobranchs retain the most primitive condition. The 

 major changes are: (1) the first arch forms the carotid arteries, 

 and (2) the more posterior arches break into capillaries, thus 

 forming the afferent and efferent arteries from the original vessel 

 Most sharks (see Squalus, page 223) are further modified by the 

 fusion of vessels, so that only three leave the ventral aorta in 

 the adult condition. 



The urodele amphibians show a reduction in the number of 

 arches, strongly resembling the modifications found in the dip- 



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