254 PROCEEDINGS OF THE ACADEMY OF 



Objection. — It mar be objected by those who have observed some of these 

 develoj)mental relations, that they are exhibited by certain single structures 

 onl}', and not by whole organisms. These objectors must not forget that the 

 distinctions of those groups, whi( h alone we have in one geological period 

 in a relation of near affinity, exist in single characters only ; and that it is 

 therefore infinitely probable that the higher groups, when we come to know 

 their representatives with the same completeness, will prove to be separated 

 by single characters of difference also, 



3. The following table is here introduced to illustrate the relations of groups 

 higher than the preceding. This is largely measured by the circulatory sys- 

 tem, not only as to the class relations, but also as regards orders. In its less 

 central portions it is, however, definitive of families at times.* [The reader is 

 here referred to the table commencing on p. 256.] 



If the reader will compare the history of the development of vertebrates of 

 any class or order, as those of Teleosts and the lizard by Lereboullet, of the 

 snake and tortoise by Rathke and Agassiz, and of the bird and mammal by 

 Von Baer, he will find the most complete examples of the inexact parallelism 

 of the lower types with the embryonic stages of the higher. A few points are 

 selected as examples, from the histories included in a few of the columns of the 

 table, and given at its end. 



Similar parallels may be found to exist in the most beautiful manner be- 

 tween the adult anatomy and structure of the urogenital apparatus within 

 each class of the series taken separately, as indicating ordinal relationship. 

 This department is, however, omitted for the present. 



As an example' of the homologies derivable from the circulatory system, and 

 of the use of the preceding table, I give the following relations betweea the 

 types of the origins of the aorta. f 



The single ventricle of Teleostei is no doubt homologous with that of Lepi- 

 dosteus, and that of Lepidosiren. The arleria vesicx nalatoripe, which is the 

 homologue of the A. pulmonalis of air breathers, issues in Lepidosteus from 

 the last vena branchialis, thus receiving aerated blood from the gills In Lepi- 

 dosiren it issues from the point of junction of two gilless and two gill-bearing 

 vense branckiales, thus receiving mixed blue and red blood, or blue blood 

 altogether, when the branchiae are not in functional activity. In Proteus it 

 issues from the last vena branchialis, where it receives the ductus bolalli of the 

 preceding vein, which, when the gill is inactive, becomes a gilless aorta-bow, 

 which brings it'onl}' carbonized blood, which it readily aerates in the swim 

 bladder, now become a lung. The ventricle is homologous with the preced- 

 ing. In Salamanders, where the substitution of the accessory gill arches by 

 the ductus botalli, converts the arterix and veme. branckiales into " aorta-bows," 

 the A. pulmonalis is given off from the posterior bow, and receives henceforth 

 mixed blood. In the Anura the origin is the same but nearer the heart. In 

 Gymnophidia it approaches the heart so far as to issue from the extremity of the 

 bulbus arteriosus, which is now divided by an incomplete septum, one half 

 conveying blood to the aorta roots, and the other to the A. pulmonalis. This 

 septum was already preceded by a longitudinal valve with free margin in the 

 Anura ! As if to meet the coming event, a trace of ventricular septum 

 appears at the apex within. There can now be no question of the homology 

 of the ventricles of the gar, and of the Ciecilia. But we have next the true 

 Reptilia. The Bulbus arteriosus is split externally, as it already was inter- 

 nally, but it is first represented in most Tortoises by an adherent portion, one- 



*This sketch is not nearly complete, but is published in hopes of its being useful to 

 students. It is compiled from the works of Meckel, Rathke, Barkow, MuUer, Hyrtl, 

 Briicke, Stannius and others, in connection with numerous dissections. 



t Professor Agassiz (Contrib. Nat. Hist. U. S., I 285) states that the ventricle of the Tes- 

 tudinata " is not any more identical with the one ventricle of fishes, than with the two 

 ventricles of warm blooded vertebrata; for in fishes we find only one vessel, the aorta, 

 arising from it, while in Turtles both the aorta and arteria pulmonalis start together from 

 it." We think this statement, which, if true, is destructive to the asserted homologies of 

 the circulatory system, cannot be substantiated, for the reasons above given. 



[Oct. 



