in the Foetus of Vertebrated Animals. 91 



all, or even of the more important bloodvessels of the body, 

 besides being too extensive a subject for our present limits, would 

 prove uninteresting from the want of connexion existing between 

 the facts already ascertained. I shall therefore confine myself 

 for the present to one branch of the subject only, viz. the De- 

 velopment of the Bloodvessels more immediately connected with 

 Respiration in the foetal or adult animal. This branch of the 

 subject, besides being the most nearly allied to that treated of in 

 the first part of the essay, is rendered one of the most interesting 

 to comparative anatomists, not only by the diversity of the form 

 and by the number of the organs which appear to carry on the 

 respiratory function in the foetus of vertebrated animals, but also 

 by the singular analogies in the structure of these animals which 

 the study of the development of their respiratory organs points 

 out both in their transitory and permanent condition. 



The principal organs which appear to perform a respiratory 

 function in the foetus, or which, being formed before birth, are 

 destined for the respiration of the adult animal, may be enume- 

 rated in the following order, being that in which they succeed 

 one another, either in individual animals, or in the different 

 orders of the class Vertebra ta. 1. The sac of the Yolk ; 2. The 

 External Gills ; 3. The Internal Gills ; 4. The AUantois ; 5. 

 The Placenta; 6. The Lungs*. 



• Some of these, as well as other parts of the ovum, have received so many 

 different names, from the various authors who have described them, that it 

 appears necessary to anticipate a little, and to give a (ew of the synonymes 

 by which they are generally known in the different orders of vertebrated ani- 

 mals. 



1. The sac of the yolk is generally known by this name in Fishes, Rep- 

 tiles, and Birds. "We have only in these animals to guard against confound- 

 ing the sac of the yolk or covering given to this part by the layers of the 

 germinal membrane, with the proper envelope of the ydk which exists before 

 development commences, and encloses it while in the ovarium. In Mammalia, 

 this part is most frequently called the Umbilical Vesicle, and sometirae3 the 

 tunica erythroides. The distinctive character of the sac of the yolk is, that 

 it remains connected or communicating with the intestine during some period 

 of foetal life, and has mesenteric arteries or veins, or both, ramified on its 

 surface. 



2. The AUantois, (a name derived originally from the vesicular membrane 

 of mammalia) does not exist in the foetus of aquatic animals, such as that 

 of fishes and batrachia. In adult batrachia it forms the urinary bladder, 



