U THE ANATOMY OF YERTEBRATED ANIMALS. 



blast, behind the alimentary tract ; but soon takes the form 

 of a vesicle, and receives the ducts of the primordial hidneys^ 

 or Wolffian bodies. It is supi^lied with blood by two arteries, 

 called Jiypogastrlc^ w^hich spring from the aorta ; and it varies 

 very much in its development. It may becom.e so large as to 

 invest all the rest of the embryo, in the respirator}^ or nutri- 

 tive, processes of which it then takes an important share. 



The spUtting of the ventral laminse, and the formation of a 

 pleuroperitoneal cavity, appear to take place in all Vertebrata, 

 Usually, there is a more or less distinct umbilical sac ; but in 

 fishes and AmijJiihia there is no amnion ; and the allantois, if 

 it is developed at all, remains very small in these two groups. 



Reptiles, birds, and mammals have all these foetal append- 

 ages. At birth, or when the ^^^ is hatched, the amnion 

 bursts and is thrown off, and so much of the allantois as lies 

 outside the walls of the body is similarly exuviated ; but that 

 part of it which is situated within the body is very generally 

 converted, behind and below, into the urinary bladder, and, 

 in front and above, into a ligamentous cord, the urachus, which 

 connects the bladder with the front wall of the abdomen. The 

 umbilical vesicle may either be cast off, or taken into the in- 

 terior of the body and gradually absorbed. 



The majority of the visceral clefts of fishes and of many 

 Ampliihia remain open throughout life ; and the visceral 

 arches of all fishes (except A'mp)hioxns), and of all Ariiphibia, 

 throw out filamentous or lamellar processes, which receive 

 branches from the aortic arches, and, as hranchirje, subserve 

 respiration. In other Vertehrata all the visceral clefts become 

 closed and, with the frequent exception of the first, obliterated ; 

 and no branchiae are developed upon any of the visceral arches. 



In all vertebrated animals, a system of relatively or abso- 

 lutely hard parts affords protection, or support, to the softer 

 tissues of the body. These, according as they are situated 

 upon the surface of the body, or are deeper seated, are called 

 exoslceleton^ or endosheleton. 



The Vertebrate iLndoslceleton. — This consists of connective 

 tissue, to wdiich cartilage and bone may be added in various 

 proportions ; together with the tissue of the notochord and 

 its sheath, which cannot be classed under either of these heads. 

 The endoskeleton is distinguishable into two independant por- 

 tions — the one axial, or belonging to the head and trunk ; the 

 other, appendicular, to the limbs. 



The axial endoslceleton usually consists of two sj'stems of 



