xl • Introduction. 



development reflected upwards from the medio-ventral line, so as to 

 meet along- the medio-dorsal line, and form thus the foetal envelope 

 known as the Amnion ; or as no such envelope is superadded to the 

 more or less complex ones, furnished by the maternal organism. 

 In the Vertebrata Amniota, a second foetal envelope, the Allan tois, is 

 always developed, originating from the anterior aspect of the poste- 

 rior extremity of the trunk as a body, which is at first bilobed and 

 solid, but which subsequently becomes hollow internally, and covered 

 externally with vascular ramifications, whereby in Reptiles and 

 Birds the respiration, and in Mammals both the respiration and 

 the nutrition of the developing* embryo are provided for. From 

 their possession of this structure, the Amniota are also known as 

 'Allantoidea ;' and as gills are never developed upon their branchial 

 arches, they are also called ' Abranchiata,' whilst the Anamniota 

 have in their turn the two additional names ^ Anallantoidea^ and 

 ' Branchiata,'' as never developing an Allantois, at least beyond 

 the stage of a urinary bladder, into which its proximal portion is 

 converted in the higher Vertebrata, and as always developing 

 either deciduous or permanent gills. 



Division, AUantoidea. 



The AUantoidea comprise the three Classes, Mammalia, Aves, and 

 Beptilia, and possess the following characteristics distinguishing 

 them from the Anallantoidea, in addition to those which their several 

 names given above connote. The axis of their basi-cranial bones 

 always forms a considerable angle with the axis of their vertebral 

 column ; the parasphenoid, which is large in the Anallantoidea, is 

 in them rudimentary, whilst the basi-occipital and basi-sphenoid are 

 always well ossified ; and the former is never anchylosed with any 

 of the anterior vertebrae, of which more or fewer are always dis- 

 tinguishable as cervical, from a thoracic or thoracico-abdominal 

 series. Inferiorly-placed ' sternar bones ordinarily complete the 

 costal arches. They never have a permanent muscular bulbus 

 arteriosus ; but they always have a trachea, and a secondary kidney. 

 They never have more than five branchial arches ; and it is only in 



