YI HEAET OF REPTILES 391 



The hearts of Eeptiles in general agree closely in their develop- 

 mental features with that of Zacerta. The most important varia- 

 tions are seen in the Crocodiles (Hochstetter, 1906*). The conus 

 here is of interest in that it still repeats with particular clearness 

 the sharp double flexure seen in the Dipnoan (Fig. 185). 



The ventricular portion of the heart becomes completely divided 

 into a right and left ventricle by a septum which is formed for the 

 most part from trabecular projections of the myocardium but in 

 part also from the endocardiae bridge which divides the atrio- 

 ventricular opening as in the Bird. This septum becomes quite 

 complete, the interventricular foramen which exists for a time 

 closing up and the interventricular septum becoming continuous 

 with the aortic septum of the conus. This is rendered a possible 

 physiological arrangement in the crocodile by the fact that here the 

 opening from the ventricle into that cavity 

 of the conus which is continuous with the 

 right systemic aorta, is farther to the left 

 than in Lizards, while on the other hand 

 the right atrioventricular opening is farther 

 to the right. The result is that the opening 

 of the right systemic aorta and the left 

 auriculoventricular opening lie to the left 

 of the septum, while the openings of the left 

 aorta and pulmonary artery together with 

 the right auriculoventricular opening lie to FlG - 185.— Heart of embryonic 

 the right of the septum. Such differences S^uoerj" 1 " H °° h " 

 in the position of the ventricular openings ' 



„ , x . , ^ . F ^.^ ? c t conus arteriosus ; V, ventricle. 



of the great arteries are regarded by Hoch- 

 stetter as due to varying degrees of incorporation of the obliquely 

 placed conus into the ventricle. 



The foramen of Panizza, that remarkable communication which 

 exists in the crocodile between the two systemic aortae close to their 

 point of exit from the ventricles, arises as a secondary perforation of 

 the aortic septum comparatively late in development just before the 

 closing of the last remains of the interventricular foramen. 



The splitting up of the conus into independent vessels remains 

 as a rule in the Eeptiles, as in Lacerta, in an incipient condition, 

 indicated merely by a slight grooving of the surface. In Ophidia 

 however the superficial groove becomes so deepened as to split the 

 conus completely into an independent aortic and pulmonary root as 

 in Birds. 



Passing in review the broad features of heart development in the 

 lower Vertebrates the following general principles seem to emerge : — 



(1) The primitive heart or primitive cardiac tube is that portion 

 of the ventral or subintestinal vessel included within the limits of 

 the pericardiac coelome. 



(2) Annular segments in the wall of this tube lag behind in 

 increase of diameter so that the tube becomes constricted into a 



