235 



ZOOLOGICAL COLLECTIONS. 



On the Existence of Vascular Arches in the Foetus of Mammifera^ Birds, and 

 Reptiles, similar to the Branchial Arteries in Fishes and the Larvce of the 



Batrachian Reptiles (With a Plate.) — It is well known that the batra- 



chian reptiles, (such as the frog, toad, water-newt, &c.) have the organization 

 and habits of a fish during the first period of their existence, and that, by a 

 metamorphosis, they afterwards assume the structure of the perfect reptile. 



Among the organs which we should expect to undergo the most remark- 

 able changes during this metamorphosis, are those of circulation and 

 respiration ; and accordingly, we find that in the tadpole, as in the fish, there 

 is but a single heart, consisting of an auricle and a ventricle, (PI. VII. fig. 1. 

 X a,) the former of which receives the venous blood from the system, and 

 the latter sends it by the branchial arteries to the gills. From these organs, 

 when it is aerated, it is returned by vessels, which, uniting together, form 

 the aorta ; by the subdivisions of which, the blood is carried to the different 

 parts of the body. The organs of respiration in the tadpole are gills, in 

 which the blood seeks the aerating meium ; in the perfect animal, lungs, 

 in which the air seeks the blood. 



The organization and habits of the higher animals are so very different, 

 even from the time of their birth, that it might be at first supposed that the 

 analogy was far-fetched which went to shew a correspondence between the 

 gills, and the arteries distributed to them, in the fish and tadpole, and any 

 structure in the mammifera, birds, or higher reptiles. This, however, is not 

 the case ; the analogy, on the contrary, is close and striking, as the researches 

 of the German physiologists into embryogenesis have fully shewn: thus, in 

 the early period of the development of the foetus of the mammifera, birds, 

 and higher reptiles, there are, (as was mentioned in a former number of 

 this Journal, Vol. II. p. 173,) the rudiments of external gills ; and there is a 

 disposition of the great arterial trunks, similar to that of the branchial 

 arteries of the fish and tadpole, (PI. VII. fig. 2, 3.) 



The analogy is still more complete if we compare these rudimentary 

 structures of the mammifera, birds, and reptiles, with the similar structures 

 in the fish and tadpole, at the early period of their development ; for, as 

 they become more developed in these latter, they, of course, resemble less 

 the rudimentary structure in the former. From this, amongst other consi- 

 derations, the transcendental anatomists infer, that the vertebrated (if not 

 the invertebrated) animals are constructed according to the same type or plan j 

 and that the higher animals, before arriving at their ultimate degree of deve- 

 lopment, successively run through stages in which their structure is similar 

 to that of animals of less complex organization. This certainly is the case 

 with individual organs; but it is properly remarked by Meckel, that, as 

 all the organs are not at the same time in the same relaiive degree of 

 development, we cannot say that a mammiferous animal is at any time a 

 fish, a reptile, or a bird. From this gradual process of formation, we can 

 understand the production of monsters, some of which have been shewn by 

 St Hilaire to be caused by a stoppage of the development of some of their 

 parts ; so that a monster from among the inferior animals having a malforma- 

 tion of the brain, never has it so complicated as that of man, although, on 

 the contrary, the ill formed brain of a human monster may resemble that of 

 an inferior animal. Other monsters are supposed to be formed by excess 

 of development, but these are comparatively rare in the more perfect 

 animals, and must arise principally from a loss of balance in structures, 

 which are merely rudimentary in the perfect state of the animal. 



