NATURAL SCIENCES OF PHILADELPHIA. 267 



and perch of the Teleosts, with that of a Lacerta among reptiles, has failed 

 10 point out characters by which the embryos of the two vertebrate classes 

 essentially differ, for a considerable period. It is true, that as each and all of 

 the species belong to widely different generic series, parallelism is of the kind 

 to be called inexact or remott. But enough is known of embryology and pala'- 

 ontology to render it extremely probable that the historic predecessors of 

 the types whose embryology Lereboullet studied, formed a series of parallels of 

 the kind termed in this essay exact. 



Lereboullet states that a certain difference exists between the eggs of the 

 fishes and those of the Lacerta. This is for us merely stating that the parents 

 of the embryos differ, a fact which no one will contest, the same may be 

 said of the elerated or depressed character of the surface of the vitellus on 

 which the embr^vo reposes, 



Secondly, after the appearance of the embryo the Lacerta is furnished 

 with the amnios and allantois, the Teliost not. This is certainly neither a 

 generic, ordinal nor class character of the adult, for it is but temporary ; 

 therefore in generic, ordinal and class characters the embryos of the Teliost 

 and Reptile are still identical. It is a physiological character and not mor- 

 phological, and therefore far the less likely to be a permanent one, eyen in 

 embryos, under changed circumstances. The female of one of the species of 

 Trachycephalus inverts the skin of the back at one season of the year to re- 

 ceive her eggs, because she cannot lay them in the water ; the other species of 

 the genus do not. The next genus in direct morphological line possesses a 

 single species whose female does the same for the same reason ; but the rela- 

 tions of these species and genera are zoologically the same as though this 

 modification did not occur. Many such instances will occur to many natu- 

 ralists. It is not pretended that they are as important as the presence of the 

 allantois :, but they constitute a character no doubt similar its kind, and en- 

 tirely at the service of the needs of the great system of morphological suc- 

 cession. The same may be said of the vascular area of the Reptile. 



Lereboullet concbides his summary of the differences between the Teliost 

 and Reptile, up to the period of completion of the heart, by saying " It is easy 

 to perceise that all these differences, however important they may appear, are 

 constituted by the accessorj- organs of the eaabryo, and do not modify the 

 "development of tlip latter, which progresses in reality exactly as in the fishes.'' 

 He says the same preriouslj^, as to the relation ef the same to the bird and 

 mammaJ- 



We have then in the embryos of the lower vertebrates at a certsJn time in 

 the history of each, an " exact parallelism " or identit;/ with tlie embryonic 

 conditioii of the type which progresses to the next degree beyond it, and of all 

 the other types which progress successively to more distant extremes. 



We have, however, so far, every reason to suppose that the embryos of the 

 •other branches of animals nes^er present au enact parallelism with tho&e of 

 .the vertebrata. 



The embryo of the fish and that of the reptile and iKammal may be said to 

 be generically if not specifically identical up to the point where preparation 

 •for the aerial respiration of the latter appears. They each take different lines 

 at this point. The fish diverges from the course of the rejitile and proceeds 

 to a different goal- the shark dees the same, but proceeds a shorter distance, 

 while the Dermopter scarcely leaves the poiist ef departure. No doubt there 

 iiave been types which never left this point, and whose plan &r circul'Uori/ si/stem 

 as identical with that ef the embryo Reptile and Mammal. Such c l?fpe was only 

 (fenerically different from the reptile or mammal which had only taken the succeeding 

 ■step, provided other structures were n^tsuper-added. 



By consparing the development of types of different classes in certain features 

 which are only ordinal or generic in meaning, very erroneous conclusions may 

 foe reached \>y the inexact student, as to the want of parallelism of classes t* 

 each other. Thus Rathke says of the development of the eye of the Tri?pi- 



1868.J " ^ ." 



