NATURAL SCIENCES OF PHILADELPHIA. 255 



half being the now, to this point, independent arteria pidmonal is hnd the otlier 

 the nearly split aorta roots. There can, I think, be little question of the exac- 

 titude of the homology throughout. 



It is no less certaiu that the Salamander* fulfils in its development the dif- 

 ferent stages to its permanent one, and is identical in each stage, in respect to 

 this point, with the orders it represents at the time. This is true even of the long 

 period during which it bears the long branchial appendages and contained 

 arteries and veins which are not found in fishes ; it is then like the Pro- 

 topterus, which has hyoid venous arches and appendages of those arches 

 at the same time. The Tortoise -\ and Tropidonotus, J are also identical in 

 their successive stages with the tj'pes already enumerated, the external or 

 appendicular branchial vessels being omitted as belonging to the special serial 

 development of the line of air-breathing Anallantoidans. Tlie division of the 

 bulbus arteriosus into three instead of two may indicate a case of inexact par- 

 allelism, but on the other hand it may be that the pulmonary partition is com- 

 pleted a little before the aorta-root partition, thus passing through the Batra- 

 chian permanent type. For explanations of inexactitude see under Part 11. 

 No doubt the Batrachian type of bulbus arteriosus is passed by many serpents 

 less extreme and specialized than the Tropidonotus. 



The aortic and pulmonary divisions of the bulbus in the Cfecilia are not 

 laterally placed, but one is dorsal and the other ventral, the one passing a 

 little spirally to the right of the other. So the pulmonary division of the 

 bulbus turns over to the right in the Anura. When the septum of the true 

 reptiles appears it rises on the anterior wall of the ventricle till it is seen in 

 Eunectes to meet the partition between the arteria pidmonalis and aorta-roots, 

 and we have at once the right and left ventricles of the bird and mammal 

 structurally and functionally. Thus are the two ventricles of man the same 

 as the one ventricle of the fish, merely divided by a septum.^ 



In the fissure of the aortic bulbus in the reptiles a spiral turn is again given, 

 and in Testudo the one aorta-root issues behind the other. In the Crocodile 

 the turn is still greater, and the right aorta-root issues to the left of the left 

 root, and vice versa. In the birds we have lost the left root, and parallelism 

 ceases with this change. In the mammalia the right root turns to the left, 

 so that in the comparison of these classes the rule of Von Baer above quoted 

 is true ; no mammal at present known is identical in a foetal stage with any 

 fully grown bird, but with a foetus of the same, up to a certain point. But 

 for both classes the parallelism of those below them holds true. 



But it is with the exact parallelism or identity of genera that we have to do 

 in the present essay. That being established, the inexact parallelism between 

 the modern representatives of higher groups, follows by a process of reduc- 

 tion. 



S. The extent of parallelism. 



Prof. De Serres and others have stated it as their belief that the lower 

 " branches " of the animal kingdom are identical with the undeveloped ibrms 

 of the higher; i. e. that the mollusc and articulate are not merely parallel 

 with, but the same as the lower conditions of the vertebrate. The works of 

 various embryologists as Von Baer and Lereboullet, have shown this state- 

 ment to be erroneous " and founded on false and deceptive appearances." The 

 embryos of the four great branches of the animal kingdom appear to be dis- 

 tinct in essential characters, from their first appearance. But Lereboullet, who, 

 in his prize essay, has compared with care the developm»nt of the trout, pike, 



* Amblystoma. f Agassiz. X Rathke. 



g Asassiz, 1. o., denies the homology of the ventricles of the turtle .ind mammal, but it 

 appears to me erroneously. He says : " The fact that the great blood vessels (aorta and 

 art. pultnonalis) start together from the cavum venosum seems to prove that the two cavi- 

 ties in the heart of turtles, which are by no means very marked, do not correspond to the 

 two ventricles in mammalia and birds." 



1868.] 



