THE VISCERAL ARCHES AND CLEFTS. » 



whicli unites the iimbilical sac with, the cavity of the future 

 intestine, becomes narrowed and elongated into the vitelline 

 duct, and as the sac itseK diminishes in relative size, the 

 amnion, increasing in absolute and relative dimensions, and 

 becoming distended with fluid, is reflected over it (Fig. 1). 



A third fcetal appendage, the Allantois, commences as a 

 single, or double, outgrowth from the under- surface of 

 the mesoblast, behind the alimentary tract ; but soon 

 takes the form of a vesicle, and receives the ducts of 

 the primordial kidneys, or Wolffian bodies. It is suppUed 

 with blood by two arteries, called hypogastric, which 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 nutritive, processes of which 

 it then takes an important share. 



The splitting of the ventral laminae, and the forma- 

 tion of a pleuroperitoneal cavity, appear to take place in 

 all Vei-tebrata. Usually, there is a more or less distinct 

 umbilical sac ; but in fishes and Amphibia there is no 

 amnion ; and the aUantois, if it is developed at aU, remains 

 very small in these two groups. 



Reptiles, bii'ds, and mammals have aU these fcetal appen- 

 dages. At birth, or when the egg is hatched, the amnion 

 bursts and is thrown ofi", 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 umbdical vesicle may either be cast 

 off, or taken into the interior 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 aU fishes (except Amphioxus), and of all Amphibia, 

 throw out filamentous or lamellar processes, which receive 

 branches from the aortic arches, and, as branchio', subserve 



