196 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



blood from the body generally, opens into the right, 

 and the lung vein into the left compartment ; in all 

 the higher forms, however, the two halves are more 

 or less connected during embryonic life, and just as 

 the tadpole has only a single auricle, so even in man 



there is a communication between 

 the right and left sides (foramen 

 ovale of the interauricular sep- 

 tum), which, in exceptional cases, 

 remains open in the adult con- 

 dition, and thereby produces the 

 affection known as cyanosis. In 

 the Mammalia the sinus venosus, 

 which in Ceratodus (Lankester), 

 though not in most lower Verte 

 brata, is not sharply separated off 

 from the auricle, becomes, when 

 foetal development is over, com- 

 pletely merged into the right 

 au.ricle (atrium or sinus venosus of 

 human anatomy). 



The ventricle remains an un- 

 divided chamber throughout the 

 Amphibia and all Reptiles except 

 the Grocodilia, so that it is clear 

 that the presence of two ventricles 

 in Crocodiles, Birds, and Mammals, 

 is not a homogenetic, but a homo- 

 plastic arrangement. (Compare page 12.) An in- 

 teresting example of the " falsification of the embryo- 

 logical record," is afforded by the development of the 

 ventricles, inasmuch as in those forms where they are 

 distinct, they become so before and not after the 

 auricles ; it is a case of what Haeckel calls cenogeiiy, 

 and is, no doubt, dependent on the requirements of 

 the organism. 



The ventricular is separated from the auricular 



Fig. 83. Heart 

 Squatina. 



of 



A, Auricle; B, arterial 

 cone ; aaa, branchial 

 arteries ; o, orifice of 

 ventricle, v. (After 

 GegenbaurJ 



