458 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



numerous cases the natural selection value of the colouring 

 means of making the egg less conspicuous is obvious. 



In Reptiles the tertiary envelopes resemble those of Birds though 

 in many cases, as in various Lizards and Turtles, there is no definite 

 rigid shell. On the other hand there may be a certain amount of lime 

 deposited in the outer layers of the shell -membrane. The albumen 

 varies in amount : inSphenodon it forms only a very thin layer (Dendy, 

 1899). 



In Elasmobranch fishes the egg is again enclosed in a layer of 

 albumen and this in turn surrounded by a shell. The shell is of a 

 horny consistency and is frequently rectangular and pillow-shaped. 

 Characteristic differences are found in different genera and species. 

 Thus in the Skates (Raia) each angle is prolonged so that the egg 



Fi<;. 207. Egg of Scylfiin/i, held in position by its four elastic filaments being wound 

 round a plant. (Figure by Kopsch, from The Cambridge Natural History. } 



has an outline like that of a hand-barrow. In Scyllium (Fig. 207) 

 the prolongations become long spirally coiled anchoring filaments : 

 in Pristiurus two short prolongations occur at one end while the 

 other end is simply rounded. . 



II. MODIFICATIONS OF THE ENVELOPES AND OTHER ADAPTIVE 

 MODIFICATIONS OCCURRING DURING THE EARLY DEVELOPMENT OF THE 

 AMPHIBIA. The Amphibians form a group of Vertebrates which 

 have taken less or more completely to a terrestrial existence in their 

 adult condition. They have not been able to emancipate themselves 

 entirely from the ancestral aquatic habitat, possibly on account >t 

 the feeble il-v-ln|.meiit of the horny outer layer of the epidermis. 

 'l'li -y are still as a rule (3 nti rely aquatic during the early st a 

 fcheir development, the eggs being laid in \vater and bhe young animal 

 passing it I ce in the water. 



In a number of cases, part ieularly in Anura inhabiting tr>pieal 

 region- with a well-mai -krd dry season, very interesting adaptations 



are found wheivby tin- \niin- animal is enabled to pass a inmv ,.r 



:<r<,l'.n;_ed period out of tin- \\atri. In the first type of these 



