. Outlines of zoology. Zoology. 588 REPTILES. History of Crocodilians.—These giant reptiles form a decadent stock. Fossil forms are found in Triassic strata ( 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 chap
. Outlines of zoology. Zoology. 588 REPTILES. History of Crocodilians.—These giant reptiles form a decadent stock. Fossil forms are found in Triassic strata ( 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 () around embryo (e.) ; , pleuroperitoneal cavity ; >\,yolk. Further growth of amniotic folds {) over embyro and around yolk. Fusion of amniotic folds above embyro ; «./., amnion proper ; $.*.,subzonal mem- brane ; y.
Size: 800px × 3123px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
