CLASSIFICATION OF CROCODILIANS 717 



(6) The Alligators, of the genus Alligator, are, with the exception of 

 one Chinese species, confined to N. and S. America. In N. America, 

 A. mississippiensis, in S. America, A. sclerops, is common. 



(c) The Gavials or Gharials, of the genus Gavialis, are distinguished 

 b}^ their long narrow snout. In the Ganges and its tributaries, G. 

 gangeticus, said to attain a length of 20 ft., is common. They feed 

 chiefly on fishes. " Old males have a large cartilaginous hump on the 

 extremity of the snout, containing a small cavity for the retention 

 of the air, by which means these individuals are enabled to remain 

 under water for a longer time than females or young." 



History of Crocodilians. — These giant reptiles form a decadent 

 stock. Fossil forms are found in Triassic strata [e.g. Belodon, Para- 

 suchus, and Stagonolepis) ; their remains are abundant in J urassic rocks. 

 In Cretaceous strata, crocodilians with procoelous vertebrce first occur, 

 the pre-Cretaceous forms having centra of the amphicoelous type. 

 The oldest crocodilians have the posterior nares situated farther for- 

 ward, behind the palatines. Huxley has worked out an " almost 

 unbroken " series from the ancient Triassic crocodilians down to those 

 of to-day. 



Development of Reptiles 



The ovum contains much yolk, at one pole of which there is a small 

 quantity of formative protoplasm surrounding the germinal vesicle 

 The segmentation is necessarily rneroblastic and discoidal, as in 

 Birds. 



The segmented area or blastoderm, originally at one pole, gradually 

 grows round the yolk. The central region of the dofsal blastoderm 

 is separated from the yolk by a shallow space filled with fluid, and 

 is clearer than the rest of the blastoderm. In this central region or 

 area pellucida, the germinal layers and subsequently the parts of the 

 embryo are established, while the rest of the blastoderm — the area 

 opaca — simply forms a sac round the yolk. One of the first signs of 

 development is the appearance of a thickened band of cells extending 

 forward in the middle line from the posterior margin of the area 

 pellucida. This band is called the primitive streak, and seems to 

 represent a fusion of the two edges of the blastopore behind the future 

 embryonic region. The embryo develops in front of the primitive 

 streak, and one of the first signs of its development is the formation 

 of a primitive or medullary groove in a line" with the primitive streak. 

 As development proceeds, folds appear around the embryo, constrict- 

 ing it off from the subjacent yolk or yolk-sac. 



It is with Reptiles that the series of higher Vertebrates or Amniota 

 begins. It is here that the foetal membranes known as amnion and 

 allantois are fi.rst formed. 



{a) The Amnion. — At an early stage in development the head end of 

 the embryo seems to sink into the subjacent yolk. A semilunar fold of 

 the blastoderm, including ectoderm and mesoderm, rises up in front. 

 Similar folds appear laterally. All the folds increase in size, arch up- 

 wards, and unite above, forming a dome over the embryo. Each of 



