SAUROPSIDA. 



291 



FIG. 290. Human embryo 



ribs are developed in connection with the transverse processes, 

 and the skeleton of the limbs (when present) is reducible to the 

 pentadactyl type. A sternum is present except in some apodal 

 forms, and is developed in connection 

 with the ribs ; and the branchial arches 

 are much reduced and modified. 



Gill pouches occur, and some of 

 these may break through to the ex- 

 terior ; but in no case are gills devel- 

 oped in connection with them, and 

 they never serve in connection with 

 respiration. The alimentary canal 

 either terminates in a cloaca, or the 

 vent is behind the urogenital open- 

 ings. The heart always has two au- 

 ricles, the sinus venosus becoming 



included in the right of these, while with the floor of the mouth 

 the ventricle, either partially or com- removed > after Hertwig. 6, 



, . . T i i i T i branchial clefts; s, cervical 



pletely divided by a longitudinal sep- sinus . ^ eye . ^ hypophysia , 

 turn, is at least physiologically divided pocket; /, lungs; , nostril, 

 into arterial and venous halves. 



In the adult true kidneys (metanephros) are developed, the 

 renal portal system is reduced or lacking in the higher forms, 

 and the posterior cardinals become greatly reduced (p. 196). 



The amniotes are divided into the Sauropsida and the 

 Mammalia. 



CLASS I. SAUROPSIDA (MONOCONDYLIA). 



Amniote vertebrates with one occipital condyle ; lower jaw 

 suspended by the free or fixed quadrate; ankle joint between 

 the first and second rows of carpals or tarsals ; coracoid well 

 developed ; external surface covered, at least in part, with ecto- 

 clermic scales ; corpus callosum rudimentary ; heart three or four 

 chambered, red blood corpuscles small, oval, nucleated ; a cloaca 

 present ; the eggs are large, and undergo a partial (meroblastic) 

 segmentation ; all except a few forms are oviparous, and the 

 eggs are enclosed in a more or less calcareous shell. 



Besides the features of the diagnosis, several other points are 



