284 CITAEACTEES OF VEETEBEATE ANIMALS. 



The next most important embryonic structures are the so- 

 called "visceral arches.'' These are a series of transverse ridges 

 developed at the sides of the embryo just posterior to the 

 head. Slit-like openings become formed in them, the "visceral 

 clefts," by -which a free communication is set up between the 

 external media and the inner part of the digestive tube 

 anteriorly. These clefts are found in all Craniota ; but they are 

 only persistent in Fish and some Amphibia, in which filaments 

 are formed on the inner parts of the clefts, the branchis or gills. 

 Although not seen in the higher adult forms, they are neverthe- 

 less always present in the embryo. 



In connection with the development there are also to be 

 noticed two embryonic membranes — namely, the Amnion and 

 Allantois (fig. 203). 



Before we can study the groups of the Craniota, it is certainly 

 necessary that we sliould know something of the typical verte- 

 brate structure. For studying the structure we will take the 

 Horse as our type, as by so doing we shall be able to compare 

 more easily the peculiarities of the various farm animals. 



