14 THE ANATOMY OF VEETEBKATED ANIMALS. 



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

 of a vesicle, and receives the ducts of the primordial kidneyn, 

 or Wblffimi bodies. It is supplied with blood by two arteries, 

 called hypogastria,'yi\A(ia spring from the aorta ; and it varies 

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

 invest all the rest of the embryo, in the respiratory, or nutri- 

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



The sphtting of the ventral laminae, and the formation of a 

 pleuroperitoneal cavity, appear to take place in aHiVertebratii. 

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

 fishes and Amphibia 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 egg 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 

 Amphibia remain open throughout life ; and the visceral 

 arches of all fishes (except Amphioxus), and of all Amphibia, 

 throw out filamentous or lamellar processes, which receive 

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

 respiration. In other Vertebrata all the visceral clefts become 

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

 and no branchise 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 

 exoskeleton, or endosheleton. 



The Vertebrate Endosheleton. — This consists of connective 

 tissue, to which 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 those 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 endosheleton usually consists of two systems of 



