REPTILES. 225 



The eggs of reptiles are various in form. In the batracliians 

 tliey are covered in the oviducts with a gelatinous substance swell- 

 ing greatly in water, in which they are developed. The eggs 

 of lizards and turtles have a leathery shell; in other tortoises the 

 shell is harder and contains more carbonate of lime, less however 

 than the eggs of birds. The membranous shell consists in most 

 cases of many layers of very line fibres, or of different films of 

 various tissue. 



In the impregnated egg of the frog the remarkable cleaving or 

 regular division of the yolk was first observed \ which precedes the 

 existence of the enibryo, and whicli we have already alluded to in 

 different invertebrate animals and in fishes, and which also occurs 

 in the egg of mammals. In the development of the animals of the 

 present class there is a remarkable difference; those reptiles which 

 have a smooth skin and at first breathe by gills, or which have both 

 lungs and gills {dti^lopnoa) , resemble fishes in the development of the 

 embryo, for neither amnion nor allantois is formed (see above p. 40). 

 In the ha^lopnoa, on the other hand, that is in those which breathe 

 by lungs from the beginning and never possess gills, as the serpents, 

 lizards, and tortoises, there is formed in the embryo, as in birds 

 and mammals, an amnion and an allantois. In the dijplo-pnoa there is 

 no external yolk-sac constricted by the abdomen; the animal leaves 

 the egg in a still very imperfect condition, as a larva without limbs 

 and with external gills, and the development continues after birth, 

 until the luno-s arise, and Avitli them the circulation of blood is 



Be'itrdfje zur Gesch. der Thierivdt, iii. 1825, s. 19 — 48, and Unfersuchungen ueher die 

 Geschlechtswerhzeuge der Schlangen, Eklechsen und ScMMkroten, Ahhaiidlungen zur 

 Bildungs- und Entwichelimgsgesch. i. Leipzig, 1832, s. 21 — 44, Taf. 2. In the tortoises 

 there is on each side of the penis near the groove a canal that terminates blindly ,• it 

 is a production of the peritoneum. In the crocodiles also there proceeds on each side 

 a funnel-shaped prolongation from the laeritoneum to the cloaca, but here it opens at 

 the base of the penis or clitoris. Comp. on these peritoneal canals IsiD. Geoffhoy 

 Saint-Hilaire and J. G-. Martin A^in. des Sc. nat. xiir. 1828, pp. 153 — 206, Mayer 

 Analecfen fiir rergl. Anat. Bonn, 1835, 4to, s. 44, 45, Tab. III. fig. 9. 



1 Prevost et Dumas Ann. des. Sc. nat. 11. 1824, p. no. See also RuscoNi 

 Dereloppement de la Grenouille commune, Milan, 1820, 4to, p. 10, PI. 2, fig. 3. But 

 hints are met with in the works of earlier observers which prove that this phenomenon 

 had not entirely escaped them, although they formed a different notion respecting 

 it from that afterwards given by Prevost and Dumas ; see ex. gr. Swammerdam Bijh. 

 d. nat. Tab. 48, figs. 5, 8. Compare on the changes of the egg and the impregnation 

 in frogs a memoir of Newport Pkilos. Transact. 1851, Part i. pp. 169 — 242. 



VOL. IL 15 



