CHAPTER XXII 



SUB- PHYLUM IV. CRANIATA 



DIVISION II. GNATHOSTOMATA 

 INTRODUCTION TO AMNIOTA 



THE three classes of Craniata, termed Araniota, agree, as their 

 name implies, in the possession of an amnion. This is a portion of 

 the egg which is transformed into a hood by which the body of the 

 embryo is enveloped, and which is thrown off at birth. But there 

 are many other features which Amniota possess in common, and 

 which suffice to show that they are all sprung from a single stem 

 of primitive land animals, distinct from that which gave rise to 

 Amphibia. All possess an allantois. This is a structure de- 

 veloped as a ventral outgrowth from the hind gut and homologous 

 with the bladder of Amphibia, but in the Amniote embryo it is 

 enlarged and extends between the limbs of the fold which con- 

 stitutes the amnion, and it functions as an embryonic respiratory 

 or nutritive organ, and this portion of the allantois is cast off at 

 birth, only the stump remaining to serve as the bladder in the 

 adult. No such modification of the bladder occurs in any Amphi- 

 bian. The vertebral column of Amniota is also formed in a totally 

 different way from that in which the vertebral column of Amphibia 

 is constructed. The basiventral arch-piece of which the rib is an 

 outgrowth does not enter into the formation of the centrum at all, 

 but forms instead an intervertebral cartilaginous pad, which very 

 rarely becomes ossified, and then forms a distinct bone called the 

 subvertebral wedge-bone. The centrum is formed from the ventral 

 intercalary piece, precisely that piece which is totally absent in the 

 vertebral column of the Frog, and only present as an element in 

 the intervertebral cartilaginous disc of Urodela. To the ventral 

 intercalary the two basidorsals forming the neural arch become 



362 



