588 



REPTILES. 



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 Jurassic rocks. 

 In Cretaceous strata, crocodilians with proccelous vertebra first occur, 

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

 Huxley has worked out an "almost unbroken" series from the 

 ancient Triassic crocodilians down to those of to-day. 



Development of Reptiles. 



As the development of Birds will be discussed in the next chapter, a 

 few notes on that of Reptiles, which is in 

 many respects similar, will be sufficient. 



The ovum contains much yolk, at one 

 pole of which there is a small quantity 

 of formative protoplasm surrounding the 

 germinal vesicle. Formation of polar 

 globules has not been observed. The 

 segmentation is necessarily meroblastic 

 and discoidal, as in Birds. 



The segmented area or blastoderm, 

 originally at one pole, gradually grows 

 round the yolk. The central region of 

 the dorsal 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 blasto- 

 derm — 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 repre- 

 sent a fusion of the two edges of the 



Fig. 256. — Origin of amnion and 

 allantois. — After Balfour. 



Rise of amniotic folds (a./.) around embryo 

 (e.) ; p.p., pleuroperitoneal cavity ; >\,yolk. 



Further growth of amniotic folds {a../.) over 

 embyro and around yolk. 



Fusion of amniotic folds above embyro ; 

 «./., amnion proper ; $.z.w*.,subzonal mem- 

 brane ; y.s.. yolk-sac. 



Outgrowth of allantois (at.); amniotic cavity 

 (a.c); k., headend: t., tail end. 



Complete enclosure and reduction of yolk-sac 

 (y.s.); s.z.m., subzonal membrane; tt.p., 

 amnion proper; «/., allantois; g., gut of 

 embryo. 



s.zm 



&V5 



